AIDS stands for "acquired immune deficiency syndrome". HIV stands for the "human immunodeficiency virus". HIV is a retrovirus that infects cells of the immune system (mainly CD4 cells and macrophages—key components of the cellular immune system), and destroys or impairs their function. HIV infection results in the progressive depletion of the immune system, leading to "immune deficiency".

The immune system is said to be "deficient" when it can no longer fulfill its role of fighting off infection and cancers. People with cellular immune deficiency are much more vulnerable to infections and to cancers such as Kaposi sarcoma. These diseases are very rare among people without immune deficiency. Some of these diseases, namely those that are strongly associated with severe immunodeficiency, are called “opportunistic infections", because they take advantage of a weakened immune system.

Speculation that HIV does not cause AIDS has in part been fuelled by the existence of groups of individuals who have been HIV-positive for many years without progressing to AIDS.

The course of HIV infection and the development of AIDS vary among individuals. About five to 10% of HIV-positive individuals develop AIDS symptoms very rapidly during the first years of infection, and about the same proportion remain infected with HIV for 15 years or more without progressing to AIDS. But on average, AIDS symptoms develop approximately eight to 10 years after initial HIV infection in people who do not receive ARV therapy.

HIV attacks two major types of cells that are important in the human immune system. The first is known as CD4 cells. These cells organize the body's overall immune response to foreign bodies and infections. The second type of cell that HIV attacks is called macrophages. Macrophages engulf and destroy infections and ensure that the body's immune system recognizes them in the future.

Once the virus has penetrated the wall of the CD4 cell it is safe from the immune system because it copies the cell's DNA, and therefore cannot be identified and destroyed by the body's defense mechanisms. Virus particles remain in these cells until their replication is triggered. Once replication is triggered, new virus particles are made. These emerge from the surface of the cell in vast numbers, destroying the cell in the process. These viruses then infect other cells.

When a person is infected a battle starts between the virus and the immune system. There is an initial burst of activity during which many cells are infected, accompanied by the immune systems attempts to fight back through making large numbers of antibodies. During this period, the viral load is high and the immune system is under attack. A person's HIV status cannot be detected using standard tests because sufficient antibodies have yet to be formed. This is commonly called "the window period" and lasts from several weeks to months. At this stage a person is highly infectious as his or her viral load (the number of virus particles they are carrying) is high. An infected person will usually experience an episode of illness at the end of the window period—but this will often resemble a simple bout of influenza and will pass unnoticed.

HIV is transmitted through sexual intercourse (anal or vaginal); blood transfusion; the sharing of contaminated needles in drug injection; and, between mother and infant, during pregnancy, childbirth, and breastfeeding. Sharing of infected blood through blood transfusion or injecting drugs is the most efficient way of transmitting HIV. The virus is not transmitted through air or water or by casual contact.

Sexual Transmission
The predominant mode of transmission of HIV is sexual. Several factors appear to influence the biological probability of transmission.

Type of Sex
The type of sexual practice affects the risk of transmission. Anal intercourse carries a greater risk than vaginal intercourse for the receptive partner. Insertive anal sex is less risky than receptive anal sex but the insertive partner can also become infected. Lesions caused by rough sex or rape can also increase the probability of HIV transmission. In the absence of aggravating factors (such as sexually transmitted infections), the virus tends to be more easily transmitted from males to females during sexual intercourse than vice versa. There is increasing evidence that the male-to-female transmission risk is higher in young girls aged 16 years and less, as compared with the risk to older women before the menopause. This may involve higher biological vulnerability because of immaturity of the genital tract, and in particular of the cervix.

There is a small chance that HIV can be transmitted through oral sex, especially if a person has abrasions in the mouth or gum disease.

Stage of Illness
The stage of illness of an infected person also influences the probability of transmission. People with HIV are more infectious to a sex partner during the earliest phase of infection (the first few weeks following initial infection with HIV) before antibodies are produced and during the later phase of the disease when the immune system is no longer able to effectively fight the virus. At both the very early and late stages of infection, a person with HIV has a very high number of viral particles in the blood. Unfortunately, many people remain unaware that a person who appears to be of perfect health could be highly infectious.

Sexually Transmitted Disease
There is scientific evidence that a person with an untreated sexually transmitted infection (STI), particularly involving ulcers or discharge, is on average, six to 10 times more likely to pass on or acquire HIV during sex. The presence of an STI means that there is more chance of broken skin or membranes allowing the virus to enter or leave the body. The very same cells that the virus is seeking to infect will be concentrated at the site of the STI because these cells are fighting the infection. According to current thinking, the risk of becoming HIV-infected from a single exposure is increased 10 to 300-fold in the presence of a genital ulcer caused by syphilis, chancroid or genital herpes (HSV-2).

Transmission via Blood and Blood Products
Transfer of contaminated blood from one person to another through blood transfusion, use of contaminated syringes or surgical equipment is the most efficient form of HIV transmission.

Mother-to-Child Transmission
HIV can also be transmitted to an infant during pregnancy, labour and delivery or
breastfeeding. Infection at delivery is the most common mode of transmission. A number of factors influence the risk of infection, particularly the viral load of the mother at birth – the higher the load, the higher the risk. A low CD4 count is also associated with increased risk. The risk of transmission varies between 15% and 30% among infants who are not breastfed. Breastfeeding increases the risk of transmission by 10-15%.

Studies from both industrialized and developing countries indicate that HIV risk does not only arise from injecting drug use. Many substances—including alcohol— affect an individual's ability to make decisions and negotiate or demand safe sex, thereby increasing their risk of acquiring and transmitting the virus. People who are drunk are less likely to use condoms than people who are sober. In one South African study, the prevalence of HIV infection was far higher among men and women who consumed alcohol than among men and women who said they never drank. Studies among men in Europe, Mexico, Zimbabwe and Uganda have also shown a strong correlation between frequent use of alcohol and other drugs, and unprotected sex.

Clinical Diagnosis
In the early stages of AIDS, when the immune system is only partly weakened, it can be hard to differentiate an ordinary patient from one infected with HIV. This is why, in the absence of knowledge of HIV status, it can take clinicians some time to make a diagnosis. Although the diagnosis of HIV continues to be simplified with improvements in HIV tests, clinicians usually suspect AIDS when patients manifest certain symptoms, suffer a series of infections and respond poorly to treatment of the individual infections. The clinical diagnosis of AIDS is therefore complex and based on core symptoms and co-presentation of "opportunistic infections."

Laboratory Diagnosis
The end of the "window period" is defined as the time when sufficient antibodies are available to be detected by current tests. Antibodies are much easier (and cheaper) to detect than the virus itself, so those types of diagnostic tests are being used widely.