The human immunodeficiency virus (HIV) is a retrovirus, a type of virus that, like many others, stores its genetic information as RNA and not as DNA (most other living beings use DNA). HIV infections can be caused by one or more retroviruses, HIV-1, or HIV-2. HIV-1 causes the majority of HIV infections worldwide, but HIV-2 causes many HIV infections in West Africa.

HIV progressively destroys certain white blood cells called CD4+ lymphocytes. Lymphocytes help defend the body against foreign cells, infectious organisms and cancer. Thus, when HIV destroys CD4+ lymphocytes, people are vulnerable to attack by many other infectious organisms. Many of the complications of HIV infection, including death, are usually the result of other infections and not HIV infection directly.

AIDS is the most severe form of HIV infection. HIV infection is considered AIDS when it develops at least one disease as a serious complication or the number (count) of CD4+ lymphocytes decreases substantially. AIDS is diagnosed when people who have been infected with HIV develop certain diseases. These diseases, called AIDS-defining diseases, include:

* Serious infections that occur primarily in people with weakened immune systems (called opportunistic infections), including fungal infections and severe herpes simplex infections
* Certain types of cancer, such as invasive cervical cancer, Kaposi's sarcoma, and certain lymphomas
* Nervous system dysfunction
* Substantial weight loss from HIV infection (AIDS consumption)

HIV transmission requires contact with body fluids containing the virus or cells infected with the virus. HIV can appear almost in any body fluid, but its transmission occurs mainly through blood, semen, vaginal fluids and breast milk. Although tears, urine and saliva may contain low concentrations of HIV, transmission by these fluids is extremely rare if it occurs.

HIV is not transmitted by casual contact (such as touch, holding the person or by dry kissing), nor by close contact and not sexual at work, at school or at home.

HIV is usually transmitted in the following ways:

* By sexual contact with an infected person
* Injection of contaminated blood, as can occur when needles are shared or a health care professional is accidentally stung with an HIV-contaminated needle
* By transmission from an infected mother to her child, whether before, during or after childbirth through breast milk.
* Medical procedures, such as HIV-containing blood transfusion, procedures performed with inadequately sterilized instruments , or transplantation of an organ or infected tissue.

HIV is more likely to be transmitted if the skin or mucous membrane is lacerated or damaged, even minimally.
 

When initially infected, many people have no observable symptoms, but within one to four weeks fever, rashes, sore throat, swollen lymph nodes, tiredness and a number of less common symptoms may arise in some people. Symptoms of initial (primary) HIV infection usually last from three to fourteen days. Some people lose weight progressively and have low fever and diarrhea.

Early diagnosis of HIV infection is important because it enables early treatment. Treatment allows infected people to live longer, healthier and less likely to pass on HIV to others.  AIDS is diagnosed when the number of CD4 drops below 200 cells per microliter of blood or when extreme cachexia occurs or certain serious opportunistic infections or cancer develop.

Currently, there is no effective HIV vaccine to prevent HIV infection or reduce the progression of AIDS in people who are already infected. However, treating people who have HIV infection reduces the risk of them transmitting the infection to others.

Treatment cannot eliminate the virus from the body, although the level of HIV often decreases so much that it is not possible to detect it in the blood or other liquids or tissues. The objectives of the treatment are: 

* Reducing HIV to an undetectable level
* Restore cd4 count to normal
 

If treatment stops, the LEVEL of HIV increases and the CD4 count begins to drop. Therefore, people need to take antiretroviral drugs for life.

Taking the medicines as instructed for the rest of your life requires a lot of effort. Some people skip doses or stop taking medications for a while (called medication vacations). These practices are dangerous because they allow HIV to develop resistance to medicines

In general, HIV infection does not cause death directly. Instead, HIV infection results in substantial weight loss (cachexia), opportunistic infections, cancers, and other disorders that in turn lead to death. Healing was considered impossible, although intensive research on how to eliminate all latent HIV from infected people continues.