CD4 cells (T cells) are the cells in the immune system that HIV attacks. CD4 cells play an important role in fighting disease. Healthy adults usually have a CD4 count of at least 800 cells per cubic millileter of blood (about one drop). HIV usually causes CD4 counts to drop; a CD4 count that falls below 200 leaves the body much more vulnerable to other infections and sickness.

Chronic infection refers to the stage of infection after recent infection. At this point, the virus has fully established itself in the body.

Drug resistance occurs when HIV mutates in such a way that HIV medications no longer work to suppress the virus. A person’s virus may become resistant to a whole group of drugs so that a new combination of HIV medications is necessary to control it.

Drug-resistance testing (genotyping/phenotyping) can show whether a person’s virus is likely to be suppressed by each available HIV medication. A genotypic test looks for genetic mutations that are linked to drug resistance. A phenotypic test assesses which drugs will stop the virus from reproducing in a test tube. A drug-resistance test may not work for someone who is not on HIV medications or has a very low viral load.

Dual infection (or coinfection) occurs when a person is infected with two strains of HIV. That person may have acquired both strains simultaneously or one after the other at the time of initial infection.

Harm Reduction is a public health philosophy or policy that has the main objective of reducing or mitigating the potential dangers and risks associated with behaviors that have inherent risks.

HIV-1 is the type of HIV that is predominant worldwide. HIV-2 is found primarily in West Africa. Within HIV-1 and HIV-2, there are at least nine genetically distinct subtypes (or clades), including A, B, C, D, F, G, H, J, and K, with subtypes B and C being the most widespread. Sometimes, two viruses of different subtypes can recombine within a person’s cells and create a new hybrid virus; for example, subtypes A and B may recombine to become an A/B mixture. Many of these new viral strains do not live for very long, but some survive to infect more than one person.

HIV medications (highly active antiretroviral therapy or HAART) are used to prevent the virus from reproducing inside the body. HAART keeps viral load relatively low, unless the virus becomes resistant to the drugs that person is taking. HAART usually consists of at least three drugs taken in combination and is also sometimes called antiretroviral therapy (ART).

HIV transmission most often occurs through sexual contact, particularly unprotected intercourse. HIV can also be transmitted through injection drug use in which contaminated needles and syringes are used, through tranfusions of contaminated blood or blood products, through the placenta from the mother to the fetus, and (rarely) through breastfeeding.

Recent infection usually refers to the first year or two of infection after seroconversion has occurred.

Sequentially expressed dual infection (SEDI) occurs when a second virus appears after a first has been established. The second virus may have been present at low levels in the body prior to its genetic appearance. On the other hand, the second virus may have been acquired after seroconversion through unprotected sex or other exposure (superinfection).

Seroadaptation refers to a wide variety of sexual choices intended to reduce the risk of HIV transmission including but not limited to avoiding anal sex, anal intercourse only with condoms, serosorting, and strategic positioning.

Seroconcordant refers to a partnership in which the partners have the same HIV status, i.e. both are HIV-positive or both are HIV-negative.

Seroconversion occurs when a person develops antibodies to HIV. This usually happens several weeks after exposure and infection. Most HIV tests are meant to detect antibodies to HIV, so an infected person will not test positive before seroconversion has happened.

Serodiscordant refers to a partnership in which one partner is HIV-positive and the other is HIV-negative.

Serosorting is the selection of sexual practices based on the partner’s HIV status. We believe that HIV-positive people frequently choose to have unprotected intercourse with other HIV-positive people rather than HIV-negative people or people of unknown HIV status. Over the past decade, men who have sex with men (MSM) in San Francisco have reported having more unprotected intercourse, but the incidence of HIV has not increased. We theorize that serosorting has allowed HIV incidence to remain stable despite this increase in what is usually considered “high-risk” sex.

Serostatus is a person’s HIV status, either negative or positive. This refers, in particular, to whether a person tests negative or positive for HIV antibodies. A seropositive individual is someone who has been infected with and has developed antibodies to HIV.

Strategic positioning is harm reduction strategy in which the HIV-positive partner bottoms to an HIV-nega¬tive or unknown-status partner during unprotected anal sex (without a condom). When the HIV-positive partner is in the receptive position there is less risk for HIV transmission to the HIV-negative partner.

Superinfection, or reinfection, occurs when someone who is HIV-positive acquires another, possibly drug-resistant strain of HIV after the first virus has already been established (see seroconversion). We know that superinfection occurs in monkeys who are recently infected, but we do not know definitively whether it happens in humans. When it looks like someone has been superinfected because a second virus appears that is genetically different from the first, it is difficult to rule out the possibility that the second virus was there, but undetectable, all along. We hope to develop more sensitive measures as well as identify source partners to more accurately characterize superinfection. If superin¬fection is rare, or if it only happens in recent infection, it is important to identify the mechanisms that make an HIV-positive person immune to acquiring a second virus. This information may aid in the development of an HIV vaccine.

Viral load is the amount of HIV in an infected person’s blood. It is usually reported as the number of copies of virus per milliliter of blood. If a person’s viral load is “undetectable,” it does not mean that there is no virus present in the blood; rather, it means that there are not enough viral copies for the test to count. A high viral load may indicate that a person’s body is not controlling the virus well, and that person may decide to go on an¬tiretroviral therapy (HIV medications). Viral load tests can also be used to diagnose HIV before seroconversion occurs.

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