How HIV Is Spread
August 25, 2008
Three conditions must be met for HIV transmission to occur:
1. HIV must be present...
Infection can only happen if one of the persons involved is infected with HIV. Some people assume that certain behaviors (such as anal sex) cause AIDS, even if HIV is not present. This is not true.
2. ...in sufficient quantity...
The concentration of HIV determines whether infection may happen. In blood, for example, the virus is very concentrated. A small amount of blood is enough to infect someone. A much larger amount of other body fluids is needed for HIV transmission.
3. ...and it must get into the bloodstream.
It is not enough to come into contact with an infected fluid to become infected. Healthy, unbroken skin does not allow HIV to get into the body; it is an excellent barrier to HIV infection. HIV can enter only through an open cut or sore, or through contact with the mucous membranes in the anus and rectum, the genitals, the mouth, and the eyes.
The next two topics discuss the second and third requirements.
Looking at the first two conditions for HIV transmission (HIV present and in sufficient quantity), let's examine some of the so-called "bodily fluids" that can contain HIV.
HIV can be transmitted from an infected person to another through:
Blood contains the highest concentration of the virus, followed closely by semen, followed by vaginal fluids. These are the three basic fluids that infect adults with HIV.
The risk from these fluids can be worsened or lessened depending on how they get into someone else's body, which we will discuss below.
Breast milk can contain a high concentration of the virus, but in this situation, transmissibility depends on who and how. An adult can ingest a small amount of breast milk at minimal risk. But an infant, with its very small body and newly forming immune system, consumes vast quantities of breast milk relative to its body weight. Therefore an infant is at risk from breast milk, whereas an adult probably is not.
HIV might be transmitted from an infected person to another through:
Pre-seminal fluid is a clear fluid that lubricates men's urethras. It is produced by a different gland than semen. Pre-seminal fluid can contain small amounts of HIV, and so there is a potential risk. However, in practice, the risk is much, much lower than that from blood, semen or vaginal fluid. As in all transmission situations, risk depends on where the fluid is going.
These fluids and substances cannot transmit HIV. Sweat contains no HIV.
The other fluids do not contain enough HIV to infect another person. This is regardless of how they get into the bloodstream. No cases of HIV transmission have ever been documented as a result of these substances. See below for more information.
Please see the sections on Reducing the Risk of Getting HIV from Sexual Activities and from Injection Drug Use for additional information.
Looking at the third condition for transmission (that it must get into the bloodstream), there are three primary ways in which this can happen:
Sexually, the three main ways that HIV is transmitted are:
Vaginal and anal intercourse: For the receptive partner (the "bottom"), the mucous membranes of the anus and vagina are very efficient routes into the bloodstream. HIV may also enter either through routine, minute tearing caused during intercourse (which is often unnoticed).
Anal sex is riskier than vaginal sex, because the membranes are thinner, tearing happens more easily, and there is no natural lubrication. But both are efficient routes for the virus.
It is speculated that for the penetrative partner (the "top"), HIV may infect through the mucous membranes in the meatus (the opening of the urethra). It is believed that uncircumcised men may be at greater risk for HIV for a variety of reasons. For instance, they may not have been told how to correctly use a condom. Because an uncircumcised man has a foreskin and a circumcised man doesn't have a foreskin, each has to put on a condom differently. Uncircumcised men must pull back on their foreskin before rolling on the condom. Neglecting to pull back the foreskin may make condom failure more likely, although no clear evidence supports this idea.
Uncircumcised men having unsafe sex may be at greater risk if HIV infected fluid remains under the penis' foreskin for an extended period of time. The shelter afforded by foreskin may give HIV a more favorable environment in which to survive. The protected area under the foreskin might help shield HIV from air and keep it closer to body temperature.
Oral sex (mouth-penis, mouth-vagina): The risk from oral sex is very minimal as the mouth is an inhospitable environment for HIV, for several reasons. Saliva contains enzymes that break down the virus; also, the skin of the mouth is sturdier than in the anus or vagina. There are, however, a few documented cases where it appears that HIV was transmitted orally. These cases are all attributed to ejaculation in the mouth (i.e., exposure to semen, not exposure to vaginal fluid or pre-seminal fluid).
Receiving oral sex is not risky because one is exposed only to saliva.
Currently, the main non-sexual ways that HIV is transmitted are:
Sharing injection needles: This is an extremely high risk behavior as an injection needle can pass blood directly from one person's bloodstream to another. It is a very efficient way to transmit a blood-borne virus. Please see the section on Injection Drug Risk Reduction for more detailed information on injected drug use and ways to reduce the risk of becoming infected with HIV.
Needle sticks: A study of over 2,000 health care workers has been underway for several years to assess the risk of their exposure to people with AIDS. Over 1,000 of these workers had a needle stick accident with a needle that had been used on a person living with HIV. The rest had some sort of mucous membrane exposure, such as being splashed in the face with blood or vomit.
Of all these people, only 21 show signs of being infected with HIV (as determined by the antibody test). One of these people was a nurse who had multiple needle stick accidents, including one where she tripped and fell on the depressor of a syringe full of blood, and the entire contents entered her body. Another was a lab worker who was working with a test tube of infected blood which broke and cut his finger, exposing the infected blood to his bloodstream. This study shows that HIV is quite difficult to get.
Blood transfusions: Blood donations in the United States have been screened for antibody to HIV type-1 since March 1985 and HIV type-2 since June 1992. This practice has almost eliminated the risk of getting HIV through a blood transfusion. Assuring the safety of the blood supply is a high-tech process requiring at least nine specific tests; proper processing, labeling, and storage; and vigilant quality control. Routine donations are now tested for HIV and hepatitis C through nucleic acid testing (NAT).
Hemophilia treatments: Hemophilia is a genetic disease in which people (almost all men) lack the ability to clot blood. To control the condition, hemophiliacs take Factor VIII, a clotting factor. Each dose of Factor VIII comes from the pooled blood of many donors. Currently, over 90% of hemophiliacs in the U.S. have been infected with HIV because of receiving contaminated Factor VIII in the early years of the epidemic. Factor VIII is now heat-treated to kill the virus. In addition, there are new synthetic products that do not pose any risk for HIV and which accomplish the same function.
Other blood products: Besides whole blood, platelets (red blood cells) have transmitted the virus. Current blood screening, however, should prevent all but a very, very few cases. No other blood products are suspected of transmitting HIV. Gamma globulin or hepatitis B vaccines do not transmit HIV. Gamma globulin, however, can temporarily transmit HIV antibodies, although not the virus itself. These antibodies will disappear within a few months.
Donor insemination: Donor semen is checked for HIV antibodies when the semen is collected. The semen is then frozen. The donor is required to come back after six months for a second HIV test, to confirm the initial HIV screening. The semen is not used before the procedure is completed.
The number of children reported with AIDS due to perinatal HIV transmission in the United States peaked at 954 in 1992 and declined 95% to 48 in 2004, largely because of the effectiveness of ensuring that pregnant women are encouraged to be tested for HIV and, for those who are infected, to receive treatments that can significantly reduce the risk of transmitting HIV to a newborn. The CDC reports that perinatal transmission rates can be reduced to less than 2% if women are aware that they are infected with HIV and take appropriate treatments to prevent transmission.
Perinatal HIV transmission continues to occur mostly among women who lack prenatal care or who are not offered voluntary HIV counseling and testing during pregnancy. Many of the perinatal HIV infections each year can be attributed to the lack of timely HIV testing and treatment of pregnant women.
For years, HIV infected women who were pregnant had little choice in how they delivered their baby. C-section delivery was considered the safest option in an effort to prevent HIV transmission to the baby. Today, more and more HIV infected women are delivering vaginally. There are guidelines as to who should and shouldn't have a vaginal delivery. Keep in mind the following are just guidelines and your doctor's choice may vary.
C-section delivery is recommended if:
Vaginal delivery can be done if:
If you are pregnant and have HIV, make sure you are getting the proper prenatal care throughout your pregnancy and talk to your doctor to find out if vaginal delivery is an option for you.
Breast milk can contain HIV, and while small amounts of breast milk do not pose significant threat of infection to adults, it is potentially a viable means of transmission to infants. Mothers infected with HIV should discuss the risk of HIV transmission to their baby via breast milk with their doctor.
HIV is not transmitted by mosquitoes, flies, ticks, fleas, bees or wasps. If a bloodsucking insect bites someone with HIV, the virus dies almost instantly in the insect's stomach (as it digests the blood). HIV can only live in human cells.
Mosquitoes cannot transmit HIV for two reasons:
These facts are confirmed by looking at infection patterns. In areas where mosquitoes are common and where HIV is prevalent, the distribution of AIDS cases in the population is not different from other areas. If mosquitoes transmitted HIV, they would be seeing a disproportionate number of children and elderly infected in those areas.
HIV is not transmitted through casual, daily contact. Since HIV is not transmitted by saliva, it is impossible to get it through sharing a glass, a fork, a sandwich, or fruit.
Three studies of household contacts, in the U.S., Europe, and Africa, have shown that AIDS is not casually transmitted by normal activities, even when people are in close living arrangements. All the studies examined households where someone had AIDS to see if any of the other members in that household had become infected (sexual contact was excluded). Many of these households included a small child as the one who has AIDS. These children continued to play with siblings in the manner that children play: wrestling, fighting, spitting, sharing food and clothes, and many other activities. No other member of any of the households shows any sign of being infected. This study shows that AIDS is a difficult disease to get, and that even the intimate exposure common among small children living together is not sufficient to transmit the virus.
Sterilized needles are always used in taking blood from donors, so HIV is not spread in this manner.
The chemicals used in swimming pools and hot tubs would instantly kill any HIV, if the hot water (and time) hadn't killed it already.
Humans are the only animals that can harbor HIV. People sometimes think they can get HIV from pets or other animals, because some animals carry viruses that produce similar immune deficiencies in their own species (e.g., FIV, feline immunodeficiency virus, in cats, and SIV, simian immunodeficiency virus, in some types of monkeys). However, FIV cannot be transmitted to people, nor can HIV be transmitted from humans to pets such as cats and dogs.
Transmission can only occur when a sufficient amount of HIV enters the bloodstream, through cuts or mucous membranes. These "bodily fluids" either contain no HIV or it exists in a quantity too small to result in transmission.
HIV is not transmitted by saliva. There is a great deal of evidence to support this fact. In a study of 79 men with AIDS, the virus could be found in the saliva of only one. This man had PCP, thrush, and other mouth and throat lesions. Even in this man, the level of virus found in his saliva was 10,000 times less than the level in his blood. To this study we can add the evidence of the countless numbers of people who have had saliva contact with people with AIDS or others who have been infected with HIV. This contact has occurred through kissing, sharing food, and many other means. They can find no evidence that these activities have transmitted the virus even a single time. Recent findings suggest that saliva contains an enzyme which kills HIV. Certainly there is a lot at work in the mouth combining to make it an inhospitable site for the virus: acids, enzymes, friction, dilution, air, and more.
Generally, when people ask the question, "How long can HIV survive outside the body?" they have come into contact with some body fluid that they think might contain HIV, and are worried about transmission. Almost always these questions are about casual contact, and we know the virus is not transmitted except during unprotected sex, sharing needles, or through significant and direct exposure to infected blood.
The length of time HIV can survive outside the body depends on:
In a laboratory, HIV has been kept viable (able to infect) for up to 15 days, and even after the body fluid containing it had dried. However, these experiments involved an extremely high concentration of the virus which was kept at a stable temperature and humidity. These conditions are very unlikely to exist outside of a laboratory. HIV is very fragile, and many common substances, including hot water, soap, bleach and alcohol, will kill it.
The chances of becoming infected with HIV by handling a body fluid are extremely small, because that fluid will rarely have access to a person's bloodstream. However, anyone handling blood, semen or vaginal fluids should be careful to avoid touching them with broken skin or getting them into mucous membranes (such as those around the eye).
Spills of blood should be mopped up, cleaned with soap and water, and then cleaned with bleach. For maximum safety, the person cleaning the spill should also wear latex gloves, and should wash the hands thoroughly after the cleanup.
Air does not "kill" HIV, but exposure to air dries the fluid that contains the virus, and that will destroy or break up much of the virus very quickly. The CDC reports that drying HIV reduces viral amount by 90-99% within several hours.
It should be noted that HIV can survive for several days in the small amount of blood that remains in a needle after use, because the blood is trapped where air cannot dry it out. As a result, used needles are very risky for HIV transmission; they provide a direct path into the bloodstream. Ideally, used needles should never be reused, but if they are, they should always be cleaned with bleach or alcohol before re-use. See the section on Injection Drug Risk Reduction for additional information on this subject.
Some people believe that taking antiretrovirals or having an "undetectable viral load" means that they can't transmit HIV to their sexual partners. That is not true. A person who takes HIV treatments -- even those who have "undetectable" viral load counts -- can still transmit HIV.
Effective drugs, taken properly, can significantly decrease viral load (the amount of virus in one's blood). It is known that the higher one's viral load, the more likely one is to transmit the virus and the lower one's viral load, the less likely one is to transmit the virus. But, regardless, HIV can still be transmitted. It is also important to note that viral load can change from day to day, so one can never be certain of their viral load count at a particular time. In addition, viral load tests only reflect the amount of virus in the blood, not necessarily the level of virus in other bodily fluids, such as semen and vaginal fluid.
Since 1996, the federal government has recommended that health care workers that are exposed to HIV through needle stick injuries be provided with antiretroviral medications to help prevent the possibility of HIV infection. For a short period of time after exposure to HIV, virus particles are present only in certain cells of the body. If HIV replication (copying of the virus) can be slowed during that time, the virus may not be able to establish permanent infection in the person's body. The sooner treatment is started, the more likely it is to interrupt HIV transmission. A combination of three antiretroviral drugs must be taken within 72 hours after exposure and must be taken daily for 28 days. This is generally referred to as post-exposure prophylaxis or PEP.
In 2005, the CDC issued guidelines for the use of anti-HIV therapies for individuals exposed to HIV through high risk sexual activities or needle sharing among injection drug users. The guidelines recommend this approach only in very limited circumstances:
It is important to note that PEP is not a simple "morning after pill" for those who have had unprotected sex. To be effective, a combination of three antiretroviral drugs must be taken within 72 hours after exposure and must be taken daily for 28 days.
There have recently been reports that taking certain HIV medications prior to engaging in high risk sexual activity may help to prevent you from being infected with HIV. This potential approach to reducing the risk of HIV infection is called pre-exposure prophylaxis or PREP.
Although there is some research currently taking place regarding the potential safety and effectiveness of such an approach, it is not yet known whether there is any protection from these drugs or what the long-term impact of taking such medications is for those who are uninfected. Researchers and medical experts also do not know how long or how frequently one should take this medication for it to be as effective as possible.
Although there are PREP studies occurring in both the United States and in other countries, no drug has been licensed for PREP at this time.
Given the lack of information about this strategy and its effectiveness, individuals should not rely on such an approach to protect themselves from becoming HIV infected.
Any procedure in which a needle or razor is used on more than one person involves a theoretical risk of HIV transmission because of the possibility of infected blood on the instrument. However, the risk can be reduced or eliminated through routine sterilization procedures. There have been no documented cases in the United States of someone becoming infected through tattooing or piercing.
Tattoo artists, piercers, hairdressers and barbers, massage therapists, manicurists and pedicurists, and acupuncturists are all defined by the Centers for Disease Control (CDC) as "personal service workers" (PSWs). The CDC has established universal precautions for PSWs, similar to those for health care workers, which are designed to protect both the workers and their customers from HIV and other blood-borne illnesses such as hepatitis B and C. The guidelines state that any instruments designed to penetrate the skin such as tattoo or acupuncture needles either should be used only once and discarded, or should be thoroughly cleaned and sterilized after each use.
If you are worried about the risks of such procedures, you should discuss infection control precautions with the provider. In the case of tattoos and acupuncture, you may also provide your own fresh needles to ensure sterility.
In California, establishments that provide personal services may be regulated locally, so anyone who has further concerns about the procedures in an establishment should call the local department of public health.
Transmission questions can often not be answered in a black-or-white fashion, (i.e. "this is always risky, that is never risky"). The answer will usually involve the following questions, going back to the conditions necessary for HIV transmission to occur:
The following are some examples of common situations and how the information about transmission can help you assess your risk.
A man performed oral sex without a condom on another man. His partner ejaculated in his mouth. He doesn't know his partner's HIV status.
Was the virus present? We don't know, because we don't know if the partner was infected.
Was there enough concentration? There was semen present, which has a high concentration.
Could HIV make it into the bloodstream? If HIV were present, it could enter the body by infecting mucous membranes in the mouth, or through open cuts or sores.
What was the level of risk? If the partner was not HIV positive, there is no risk. On the other hand, if the partner were HIV positive, there would be a potential risk because of the semen coming into contact with the mouth. Considering that there are few reported cases of transmitting HIV through oral sex, the risk in this situation is relatively low.
If the man is very worried about the incident, it is important for him to remember that there was some risk involved, but that the risk is not very high. If he feels that there was no risk at all and that he can continue practicing unprotected oral sex, it is important for him to know that although the risk was low, it is still there.
A woman has found out that her previous drug partner is HIV-positive. They only shared needles once. She thinks that he got infected after they stopped seeing each other.
Was the virus present? We don't know because we don't know when he became infected.
Was there enough concentration? Yes. HIV in blood may be transmitted if two people share needles.
Was there a path of infection? Yes. Sharing needles provides a direct path for HIV to reach the bloodstream.
What was the level of risk? High, if her partner was already infected when they shared a needle.
It is important for her to be aware that there is a possibility for high risk.
A woman is concerned about getting HIV from a co-worker. She accidentally drank from his cup. She thinks that he may be gay.
Was the virus present? We don't know. We cannot judge whether a person is infected or not based on his or her sexual orientation.
Was there enough concentration? No. Even if he's infected, she would have been in contact with his saliva. Saliva does not transmit HIV.
Was there a path of infection? In this case, this question is irrelevant because of the answer to the previous two questions.
What was the level of risk? There was NO risk. She cannot get HIV from drinking from the same cup, even if the person is infected.
This article was provided by San Francisco AIDS Foundation. It is a part of the publication AIDS 101. Visit San Francisco AIDS Foundation's Web site to find out more about their activities, publications and services.