Principles of Contagion

Preventing transmission begins with understanding the principles that govern how infections are transmitted. These principles are called the principles of contagion.

Infectious diseases and contagious diseases are two different things.

The microbe that causes influenza is a virus found in the nose, throat, and lungs of the person who is infected. Influenza is spread when secretions from the nose, throat, or lungs of the infected person are passed to another person. When an infected person coughs or sneezes on another person, or touches another person, these secretions and the virus they carry are transmitted.

People can be either susceptible or not susceptible to an influenza virus. If they have been infected with that particular virus or a closely related one before, or if they have been vaccinated against the virus, they already have antibodies against it, so they are not susceptible and will not get influenza. If they do not have these antibodies, they are suscep­tible and will get influenza.

Whether susceptible or not, the person will not become infected if the type of contact is wrong. Specific viruses can live only on specific tis­sues within the body. An influenza virus on the skin of your hand will not give you influenza; the same virus on the membranes of your nose, throat, or lungs will.

Given susceptibility and the right type of contact, some viruses are more likely than others to be spread from person to person, that is, some viruses are transmitted with greater efficiency than others. Some viruses are difficult to spread; others are easy. For the influenza virus, for ex­ample, even very brief contact with a person who is infected is likely to result in transmission. Highly efficient transmission accounts for the an­nual epidemics of influenza.

The efficiency with which a virus is transmitted also depends on the number of viruses a person is exposed to, or the inoculum size. Living with a person with influenza is obviously more likely to result in suc­cessful transmission than simply working with that person in the same office. And being sneezed upon poses a greater risk than passing some­one in a hallway. In short, how efficiently a virus is transmitted depends on both the number of influenza viruses and the type of contact.

A person, once infected, may continue to feel well for a day or two but, during this time, can still pass the virus to others. This early period between infection and the beginning of symptoms is called the incuba­tion period.

HIV, like influenza, follows the same general principles of conta­gion. An infected person is the source of HIV. HIV is contagious if a per­son is susceptible and the contact is of the kind necessary for transmis­sion. And HIV has a certain efficiency of transmission and a certain incubation period. There the resemblance ends.

This point deserves emphasis. Much of the misunderstanding about HIV infection has been based on the assumption that HIV is transmit­ted like other common infectious diseases. It isn’t. In brief, for HIV, the types of contact are very specific, transmission is inefficient, and HIV’s incubation period is very long.

Sources of HIV

A person with HIV infection is almost the sole source of this infection.

Any person with HIV infection, regardless of symptoms, should be considered capable of transmitting the disease.

Types of Contact

The white blood cells that HIV infects, the CD4 cells, are found in dif­fering numbers in different body fluids. As a result, the numbers of HIV also differ in different body fluids. The numbers of HIV in body fluids— the usual source of transmission—are greatest in semen, vaginal fluid, breast milk, and blood. HIV is unlikely to be in saliva, stool, or tears, though it has been found in these fluids in a minority of people, and then only in very low numbers. HIV has not been found in urine.

In order to cause infection, HIV must travel from the body fluids of an infected person into the bloodstream of an uninfected person. The skin that covers the outside of the body is a formidable barrier. If the skin is intact, simple contact between HIV and the skin will not trans­mit HIV. The mucous membranes that cover most of the insides of the mouth, vagina, and rectum are also a barrier to the virus. If the skin or a mucous membrane is broken—if it has cuts or sores—the virus can get into the bloodstream. Thus, infected blood (including menstrual blood), vaginal fluids, or semen on intact skin is almost invariably safe. But on skin or mucous membranes that have an open sore or a cut, the same fluids can possibly transmit the virus. Injecting large amounts of infected blood into the body—like a transfusion of blood from an infected per- son—is the most efficient method of transmission.

We can provide absolute assurance that most types of common con­tact carry no risk of transmitting the virus. These include a variety of ex­periences often referred to as “casual contact”: shaking hands, hugging, sharing a toilet, sharing eating utensils, closed-mouth kissing, being sneezed on, and so forth.

There are three primary types of contact that can result in trans­mission of HIV:

• Sexual contact, that is, contact with infected genital secretions (se­men, vaginal fluids)

• Injection of infected blood through transfusions or needle sharing

• Pregnancy in an infected mother—now rare because of effective preventive treatment

Other kinds of contact more rarely result in transmission of HIV. These are

• Breast-feeding by an infected mother (transmission to baby)

• Breast-feeding by an infected baby (transmission to mother)

• Organ transplantation using organs from infected donors

• Artificial insemination from infected sperm donors

• Needlestick injuries in health care professionals caring for infected people

Oral sex, either cunnilingus (oral sex performed on a woman) or fel­latio (oral sex performed on a man), can transmit HIV infection. The greater risk is fellatio: the semen of an infected man has more HIV than the vaginal secretions of an infected woman.

The combined total for these rarer types of contact accounts for about 0.1 percent of U.S. cases, actually amounting to only about 500 of the first 500,000 cases of AIDS reported to the Centers for Disease Control and Prevention (CDC). Transfusions, now screened for HIV, are excluded from this number.

We are sure about what kinds of contact do and do not transmit the virus. More than 500,000 people with AIDS have been studied by the CDC. The types of contact listed above together account for 95 to 97 percent. When researchers went back and looked specifically at the re­maining 3 to 5 percent of people not accounted for by these types of con­tact, they found that most were not problems: many people acknowl­edged risks when questioned by a more experienced interviewer; some people were so seriously ill at the time of reporting that no reliable med­ical history could be obtained; and some never had HIV infection to be­gin with.

At the same time, we must acknowledge that other types of contact, though unlikely to transmit HIV, might do so at least theoretically. HIV has been found in low numbers in saliva, stool, and tears. HIV has not been found in urine. Although transmission through these fluids is bio­logically possible, it doesn’t seem to happen; the CDC, which tracks all cases of AIDS, has no case in which the only type of contact was clearly through feces or urine or tears. There is one possible case of a bite trans­mitting HIV, and another case of presumed transmission through deep kissing, but the details of both cases are sketchy. Perhaps the inoculum size—the numbers of the virus—in these fluids is simply too low. In any case, transmission of HIV through these types of contact is extremely in­efficient and is not known to happen. Unfortunately, the CDC and other groups continue to talk about “body fluids” as the source of HIV infec­tion. This gives the wrong impression that the source of infection is all body fluids. In fact, the only body fluids that are the source of infection are semen, vaginal fluids, breast milk, and blood, including menstrual blood.

One type of contact that people worry about is mouth contact. In­fection by mouth may occur in three ways: breast-feeding, oral sex, or deep kissing. All three are biologically plausible methods of transmis­sion and all three have occurred. The three are enormously different, however, in the level of risk. Breast-feeding results in heavy exposure to infected CD4 cells in breast milk; in African countries, where the major studies have been done, the risk of transmission is 15 to 25 percent. Oral sex, or fellatio, is also a plausible risk for the same reason: semen has a large number of infected CD4 cells.

Another type of contact people worry about is indirect: becoming infected by a virus on a surface outside the body. To repeat, we are not aware that anyone (except the rare laboratory worker using high con­centrations of the virus) has ever become infected by the virus living on a surface outside the body. The reason is that HIV cannot survive out­side its host cells, and outside the body, cells die quickly. When host cells die, HIV dies with them. Although HIV can survive outside the body on a surface for up to fifteen days, the numbers of viruses on a surface fall rapidly to levels well below those necessary for infection.

A third possibility that people used to worry about, probably be­cause the news media have paid a lot of attention to it, is that insects, particularly mosquitoes, could conceivably transmit the virus. The ar­gument is that insects transmit other microbes in the blood, such as malaria. But even in Africa, where mosquito-borne diseases like malaria are common, scientists have not been able to find a clear case in which HIV has been transmitted by a mosquito. AIDS in Africa is a disease found almost exclusively in babies of infected mothers and sexually ac­tive adults, especially those in cities. Mosquitoes, however, do not select out babies and sexually active adults in cities to bite; mosquitoes are everywhere and bite everyone.

A fourth possibility is that a patient might get HIV infection from an infected health care worker, like a surgeon or dentist. In 1990 a dentist in Florida apparently infected six of his patients, though exactly how the dentist transmitted HIV infection has never been sorted out and never will be. This case raised a new possible mechanism of infection—acqui­sition from your dentist or possibly from your surgeon, presumably by mixing of blood. A second case involved an orthopedic surgeon in France who is believed to have infected one of the patients he operated on. En­suing studies show that these two cases were isolated events. Testing of about 20,000 patients who had surgery or dental care from someone with HIV infection failed to turn up a single case of HIV transmission. The paucity of cases has confirmed the incredibly small risk associated with transmission from surgeon to patient.

Efficiency of Transmission

Because the types of contact are so specific and the numbers of the virus in some body fluids are so low, HIV is not transmitted efficiently. Cer­tain kinds of contact transmit HIV more efficiently than others. This sec­tion will discuss efficiency of transmission, that is, the likelihood that a given type of contact will transmit HIV. The types of contact that trans­mit the virus most efficiently are those with the highest risk for infection.

There are four categories of risk for HIV infection, from the kinds of contact that are most likely to transmit the virus to the kinds that are least likely to do so.

1. Very likely risks. These are well-established, common ways to trans­mit the virus. They account for over 99 percent of cases. The order in which they are discussed below does not imply that there is a hierarchy of risk within this category. All these behaviors pose a high risk of in­fection.

The first high risk is nonsafe sex—that is, sex without condoms, or sex that involves exchange of body fluids—with people known to have HIV infection, or with people who have a high risk for HIV infection. The risk of getting HIV through nonsafe sex with an infected person is roughly estimated from studies done of discordant couples, couples (usually spouses) in a regular sexual relationship in which one partner has HIV infection and the other does not. These studies show that with­out condoms and without HIV treatment, the risk of HIV transmission is one per five hundred to one thousand episodes of vaginal sex.

How efficiently HIV is transmitted during sexual intercourse de­pends on a number of factors. One of the most important is whether the uninfected person has open sores on the genitals. The most common cause of genital sores are other sexually transmitted diseases such as syphilis and chancroid. The most common cause of genital sores in the United States and in the world is genital herpes. Open sores from these or other causes allow the virus to enter directly into the blood. The risk of transmission consequently increases enormously when sores are pres­ent on the genitals.

A second factor increasing the risk of transmission is the type of sex­ual practice. Virtually all sexual contact that has resulted in infection with HIV has been either anal or vaginal intercourse. Studies of trans­mission of HIV infection among gay men suggest that anal intercourse is an especially efficient means of transmission. During anal intercourse, the thin walls of the rectum are often cut or scraped, exposing blood ves­sels to infected semen, and the surface area of exposure to the virus is very large. The vaginal wall is thicker and less likely to be cut, and the vagina is short compared to the colon, so transmission is less likely but still distinctly possible. The estimated risk of HIV transmission with a single episode of unprotected anal intercourse is 1 to 3 per 100 (1 to 3 percent); the risk with a single episode of unprotected vaginal inter­course is 1 per 500 to 1,000 (0.1 to 0.2 percent). Although the 1 percent figure for anal intercourse seems low, remember that this is the presumed mechanism of transmission for one-third of all AIDS cases in the United States. In short, any sexual practice that exposes the blood of an unin­fected person to the blood (including menstrual blood) or semen or vagi­nal fluid of an infected person will allow transmission of HIV.

A third factor, and probably the most important, is the numbers of HIV, or the viral load. The idea is simply that infection is more likely if there are more viruses. People have the highest number of viruses—the highest viral loads—in the first three to four weeks after transmission. During this early stage, most people are completely unaware they are in­fected. Because viral loads are so high and because people are usually unaware they are infected, this early stage of infection actually accounts for an estimated 30 to 40 percent of all sexual transmissions. After three to five weeks, the body develops antibodies to the virus, so the viral load drops and the probability of transmission drops along with it. During the late stage of untreated HIV infection, the viral load increases again, but transmission during this stage is limited because people are sick and less interested in sex. The chances of transmission are about 1 in 100 acts of sexual intercourse in the early stage, 1 in 1,000 acts of sexual in­tercourse in the chronic stage, and 1 in 500 acts of sexual intercourse in the late stage.

It should be noted that these estimates are based on long-term stud­ies done in Africa of discordant couples without HIV treatment. Since treatment reduces viral load, it should also reduce the probability of transmission; this makes biologic sense, but it is not yet scientifically proved. A concern is that people under treatment who believe their dis­ease is unlikely to be transmitted might practice high-risk behavior: gay men do have high rates of such other sexually transmitted diseases as syphilis and gonorrhea—though the data establishing these rates are restricted to a small number of gay men in specific locations. The bot­tom line is that regardless of viral load, stage of infection, or treat­ment status, preventing sexual transmission of HIV requires no sex or safe sex.

A second very likely risk of transmitting HIV infection comes with sharing needles or works (spoons, cotton, syringes) with a person who injects drugs, especially in cities with a high incidence of HIV infection among injection drug users. HIV can be transmitted by blood left on a contaminated needle, or by blood left in the syringe, or by blood on any other components of the works used to prepare the drug for injection. The risk of HIV transmission with needle sharing is estimated at about 3 per 1,000, or 0.3 percent. Factors that influence the efficiency of transmission include the amount of blood, how long the blood has been outside the body before injection, whether the blood is dried, and how many viruses are in the blood. For people who inject drugs, the risk of getting HIV from sharing a needle with a person with HIV infection ap­pears to be about the same as the risk of having sexual intercourse with that person.

A third very likely risk is pregnancy in a woman with HIV infec­tion. An infected mother taking no anti-HIV drugs has a 20 to 35 per­cent chance of transmitting HIV to her unborn child, unless she deliv­ers her baby by cesarean section. Transmission can take place while the infant is still in the uterus or when the baby passes through the birth canal and is in contact with the mother’s genital secretions and blood. When mothers don’t take anti-HIV drugs, one-third of trans­missions happen in the uterus and two-thirds during birth. When mothers do take anti-HIV drugs, the likelihood of transmission drops; of the transmissions that do occur, about two-thirds happen while in­fant is still in the uterus and one-third during birth. The lesson here is that when the mother takes drugs to reduce HIV, the infant is less likely to become infected during birth. If the mother’s viral load is “unde­tectable,” the risk of transmission to the infant is only 2 percent. If the mother’s viral load is not under control, we usually recommend that delivery be done by cesarean section. When delivery is done by ce­sarean section, the infant’s probability of becoming infected is only 2 percent as well.

The bottom line is straightforward. Regardless of how the infection is transmitted, a person with a high viral load is more likely to transmit infection. For instance, someone with a viral load of 100,000 copies of the virus per milliliter is at least ten times more likely to transmit HIV than someone with a viral load of 100. And a low viral load in the blood reduces the risk of transmission but never eliminates it. Furthermore, no one working in this field thinks that the new treatments now be­ing developed will eliminate the risk of transmission. At best, the sit­uation is still much like playing Russian roulette, just with more empty chambers.

2. Likely risks. This section discusses the ways in which HIV may pos­sibly be transmitted. The risks are scientifically established, though ac­tual transmission in these ways is rare.

The first likely risk is safer sex with people known to have HIV in­fection or with people who have a high risk of HIV infection. Safer sex is sex with condoms, or sexual contact that does not involve getting semen, blood, or vaginal fluid from one person’s body into another’s. Though safer sex is known to be safer, no one believes it is completely safe. The failure rate of condoms—due to breakage or improper use— is 2 to 5 percent. With the proper use of latex and polyurethane con­doms, the protection is nearly 100 percent.

The second likely risk is nonsafe heterosexual sex with multiple partners. The risk is higher if partners are anonymous, injection drug users, gay or bisexual men, or prostitutes in cities with high rates of HIV infection. No one knows exactly what the risk of nonsafe sex with mul­tiple partners is; the risk depends entirely on whether your partners are likely to be infected.

The third likely risk is breast-feeding. The risk of transmitting HIV with breast-feeding is high, probably 15 to 25 percent, if done for six to twelve months. The risk appears to diminish with time but never entirely disappears. Drugs to reduce the amount of HIV in the mother and drugs given to the baby to prevent transmission are both effective, but no one knows exactly how effective. Women in developed countries can easily solve the problem of transmitting HIV infection with breast-feeding, simply by avoiding breast-feeding. In developing countries, however, the issues are more complex.

The fourth likely risk is needlestick injury, primarily for health care workers: 1 in 300 needlestick injuries has transmitted the virus to the person stuck when the source of the stick had HIV infection. If the health care worker takes AZT promptly after being exposed, the risk is reduced by 80 percent, to about 1 in 1,500. Of the 4 million health care work­ers in this country, about 57 have HIV infections from exposures to in­fected patients. Nearly all of the transmissions were caused by “sharps injuries.” A sharps injury is an injury with a sharp instrument like a nee­dle or a scalpel during which the health care worker’s blood mixes with the patient’s blood. Of every 300 sharps injuries involving patients with known HIV infection, 1 health care worker will become infected. The risk is much higher than 1 in 300 if the injury was deep, if the health care worker was exposed to a lot of infected blood, or if the infected pa­tient’s viral load was high. Without these risks, the probability of trans­mission from an infected patient is much less than 1 in 300. Other kinds of transmission to health care workers are rare. About five health care workers became infected when blood splashed into their eyes, mouth, or skin. This kind of transmission is less than 1 per 1,000 when the blood carries HIV. And it appears that rare patients may acquire HIV infection from infected health care workers as well: we know of one dentist who transmitted HIV to six patients, though just how the transmission oc­curred is unclear; and we suspect HIV transmission from an infected or­thopedic surgeon to his patient during surgery and from a gynecologist to his patient, both in France. These are the only three known cases.

The fifth likely risk is blood transfusion, artificial insemination, or organ transplantation from an infected donor: between 1978, when HIV infection first appeared, and 1985, when the Red Cross began screening all blood for HIV, this was a high risk if the donated blood contained HIV. About 90 percent of the people who received HIV-contaminated blood became infected with HIV. Now the blood supply is almost, but not completely, clear of HIV: it is estimated that about 1 in 500,000 units slips past the HIV screening process. The same scenario held for people with hemophilia, who are treated with a blood product called clotting factor, which is pooled from the blood of several thousand donors. Be­tween 1978 and 1985, about 50 percent of people with hemophilia who were treated became infected with HIV. Currently, not only are all blood donors screened, but the clotting factor is treated so that any HIV that does slip through is destroyed before transfusion. Screening donors and treating blood works very well but not perfectly. Each year in this coun­try, a small number of people get infected with HIV after receiving trans­fusions with screened blood. During the first three years after screening began, 18 people with hemophilia are known to have become infected from clotting factor; currently, the annual rate is less than 1 per 1,000. And there is one famous case of HIV being transmitted by organ trans­plantation. A man killed by gunshot donated various organs and tissues to 58 people. Though his blood test was negative, 7 of the recipients be­came infected. Probably the donor was tested in the “window” between infection and a positive blood test (see Appendix B, “Understanding Tests for HIV”). Note that this is the only case of infection among the 60,000 organs and 1 million tissues transplanted in the United States since 1985. Finally, in the United States, at least 7 women have become infected through artificial insemination, but all these transmissions oc­curred before 1985, when semen donors began to be screened. Donors are now screened, so this mechanism of transmission appears to be elim­inated.

The sixth likely risk is unprotected oral sex. No one knows just how much of a risk unprotected oral sex, or fellatio, is. People have definitely become infected through unprotected oral sex, probably because semen has heavy concentrations of CD4 cells. The problem with figuring out the risk exactly is finding people who have practiced no other risk behaviors except oral sex. Oral sex performed on a woman is much less likely to transmit HIV. Inserting the fingers into the vagina, or fisting, is unlikely to transmit HIV because no body fluid is exchanged. Nevertheless, there is a theoretical risk if the skin on the hand has cuts or abrasions.

The seventh likely risk is biting: there are several reports of people getting HIV infection from human bites, but all save one of these reports were tarnished by the coexistence of other risk factors. One estimate of the risk of transmission when the biter has HIV is 1 in 1,000. One rea­son for the low rate is that HIV is not usually found in saliva, and when it is, it is in low numbers. A much greater risk is bleeding gums, the ap­parent cause of a widely publicized case attributed to deep kissing.

The eighth likely risk is home care: people with AIDS often require extensive home care by someone with no training in the methods of con­trolling the spread of infection. In the hospital, these methods of con­trolling infection, called “universal precautions,” consist of a barrier be­tween the caregiver and such infectious material as blood, stool, or pus. Universal precautions are relatively easy to learn. Most important to know is that now hundreds of thousands of people with AIDS have re­ceived home care by loved ones, and we are aware of only eight care­givers who have become infected with HIV as a result of their care. These eight apparently took no precautions.

It is important to emphasize the infrequency of the transmission of HIV in this second category of “likely risks,” compared to the first cat­egory of “very likely risks.” Of the million-plus cases of HIV infection in this country, this second category of “likely risks” accounts for about 0.02 percent, or 1 case in 10,000.

3. Biologically plausible risks, but unlikely and unconfirmed. These types of contact might possibly transmit HIV, but they are unlikely to. They are obviously controversial because they are almost impossible to either prove or disprove. If they do transmit HIV, they do so only rarely. As a matter of fact, there are no established cases in this category at all; these risks are totally theoretical. We include them to pacify the “what- if ” crowd, the people who worry about risks that are biologically plau­sible but have no evidence to support or reject them.

Sharing toothbrushes or razors with an infected person

Exposure to body fluids (tears, urine, feces, saliva, sweat) other than genital secretions or blood

Exchange of saliva with deep kissing

Mosquito transmissions (according to studies, improbable and probably impossible; it is placed in this category simply to be con­servative)

4. Transmission that is not biologically possible. Transmitting the virus through the following types of contact is inconceivable. The first lesson learned in medical school is “never to say never”: nevertheless, these types of contact not only lack precedent but also appear absurd on the basis of our current scientific knowledge of HIV infection.

Shaking hands

Sharing a toilet

Sharing eating utensils

Being sneezed upon

Living in the same household

Working in the same room or attending the same classroom

Sexual transmission by partners known to be monogamous and un­infected

Any contact with any pet

Closed-mouth kissing

Incubation Period

For most infectious diseases, the incubation period—the period from the time of infection until the person feels the first symptoms—is a few days or weeks. HIV infection is almost unique; its incubation period is five to eight years. This long incubation period accounts for the large reservoir of persons who are infected for years before they are aware of it. Despite not having symptoms, the person infected with HIV is contagious to oth­ers throughout the incubation period.