What are HIV and AIDS?

HIV (human immunodeficiency virus) is the virus that causes AIDS (acquired immunodeficiency syndrome). HIV progressively destroys the body's ability to fight infections and certain cancers. It weakens the immune system by infecting lymphocytes, a type of white blood cell, that normally help the body fight infections. Specific lymphocytes known as T-helper cells or CD4 cells are major targets for HIV. The virus binds to CD4 cells, enters them, replicates inside them, and eventually kills them.

Over time, the amount of HIV virus—the viral load—increases while the number of CD4 cells in the blood declines. After several years without treatment, the number of CD4 cells can drop to the point that AIDS-associated conditions and symptoms begin to appear. AIDS treatments can slow disease progression by reducing the amount of HIV in the body. This allows the body's CD4 cells to increase or stabilize.

Currently, the Centers for Disease Control and Prevention (CDC) estimates that about 50,000 people in the U.S are newly infected with HIV each year, that 1.2 million people in the U.S. are living with HIV infection, and that nearly 13% of those with the infection are not aware of it and can pass the virus on to others. In 2012, the year with the most current statistics, nearly 14,000 people with AIDS died, and almost 660,000 have died since the beginning of the epidemic, according to the CDC.

Worldwide, as many as 2.1 million people in 2013 were newly infected with HIV, 1.5 million died of AIDS-related illnesses, and 35 million people were living with HIV, according to the World Health Organization.

HIV can be spread the following ways:

  • By having unprotected sex with an infected partner; the virus can enter the body through the lining of the vagina, vulva, penis, rectum, or mouth during sex. Having a sexually transmitted disease (STD) such as syphilis, genital herpes, chlamydia, gonorrhea, or bacterial vaginosis appears to make people more susceptible to and at higher risk for acquiring HIV infection during sex with infected partners.
  • By sharing needles or syringes (such as with intravenous injection drug abuse), which can be contaminated with very small quantities of blood from someone infected with the virus.
  • During pregnancy or birth; approximately 25% to 35% of all untreated pregnant women infected with HIV will pass the infection to their babies. HIV also can be spread to babies through the breast milk of mothers infected with the virus. If the mother is treated with antiretroviral therapy (ART) during pregnancy, she can significantly reduce the chances of passing the infection to her baby.
  • Through contact with infected blood; in the U.S. today, because of screening blood for transfusion and heat-treating techniques and other treatments of blood derivatives, the risk of getting HIV from transfusions is extremely small. However, before donated blood was screened for evidence of HIV infection and before treatments were introduced to destroy HIV in some blood products, such as factor VIII and albumin, HIV was transmitted through transfusion of contaminated blood or blood components. In areas of the world where donated blood is not routinely screened or treated for HIV, there is still risk of contracting the disease through this mode of transmission.

Initially, HIV usually causes flu-like symptoms, but some people may not experience any obvious signs or symptoms. The only way to determine whether a person has been infected is through HIV testing.

A person's HIV status, like other medical conditions and test results, is protected by the Health Insurance Portability and Accountability Act (HIPAA) Privacy Rule and cannot be shared by healthcare providers with friends, family, or employers without written permission. However, if a person tests positive for HIV, it is important that he or she tell their healthcare providers as well as all sex partners and/or anyone with whom they share needles. Counseling services are often available from the clinic or healthcare provider that performed the test and they can help to advise the individual on who needs to know.

HIV status may be shared with healthcare providers who have a "need to know" in order to treat an individual. Also, in order to determine the incidence of HIV and to provide appropriate prevention and care services, all new cases of HIV are reported to state and local health departments. As of April 2008, data from all 50 states, the District of Columbia, and 5 dependent areas (American Samoa, Guam, the Northern Mariana Islands, Puerto Rico, and the U.S. Virgin Islands) are collected using a confidential name-based reporting system.

Development of AIDS

HIV initially causes an acute illness with non-specific, flu-like symptoms. During this time, the virus is present in large numbers and is carried throughout the body. About 2 to 8 weeks after exposure, the person's immune system responds by producing antibodies against the virus. As HIV infects the immune cells called CD4 T-cells (also called helper T cells), it slowly begins to decrease their numbers. The person may be apparently healthy for a decade or more, but without treatment, HIV continues to replicate and destroy CD4 T-cells. The virus remains in places such as the brain and lymph nodes, where it will persist even during drug treatment.

The term AIDS applies to the most advanced stages of HIV infection. According to the CDC, AIDS is diagnosed when an individual's CD4 T-cell count drops below 200 cells/mm3. AIDS is also diagnosed when an individual has HIV and an AIDS-related illness, such as tuberculosis or pneumonia caused by the microorganism Pneumocystis jirovecii (carinii). In people with AIDS, opportunistic infections are often severe and sometimes fatal because the immune system is so damaged by HIV that the body cannot fight off certain bacteria, viruses, fungi, or parasites. Those with HIV/AIDS are also at an increased risk of developing certain cancers, neurological disorders, and a variety of other conditions.

Signs and Symptoms

HIV initially causes an acute illness with non-specific, flu-like symptoms such as fever, fatigue, headache, sore throat, rash, and enlarged lymph nodes. Some people do not experience any noticeable symptoms. In most people, the initial symptoms go away after a few weeks. The only way to be sure whether a person is infected is through HIV testing.

If HIV is not diagnosed early and treated, it infects and kills more CD4 T-cells, decreasing their numbers. The affected person's immune system may eventually become weakened to the extent that the person begins having signs and symptoms such as:

  • Persistently enlarged lymph nodes
  • Recurring fever 
  • Rapid weight loss
  • Profuse sweating, night sweats
  • Extreme fatigue
  • Persistent diarrhea
  • Rashes
  • Sores in the mouth or on the genitals or anus
  • Memory loss or difficulty concentrating

In children who are infected with HIV at or before birth, symptoms may emerge within a couple of years. Untreated children may have delayed development and be frequently ill. Pneumonia is common in children born with HIV.

If still left untreated, HIV can progress to AIDS, the most advanced stages of HIV infection. People diagnosed with AIDS may get life-threatening diseases called opportunistic infections caused by microbes such as viruses, bacteria, fungi, or parasites. These infections do not usually make healthy people sick.

Opportunistic infections common in people with AIDS cause signs and symptoms such as:

  • Coughing and shortness of breath
  • Soaking night sweats
  • Shaking chills or a high fever for several weeks
  • Seizures and lack of coordination
  • Difficult or painful swallowing
  • Mental symptoms such as confusion and forgetfulness
  • Severe and persistent diarrhea
  • Vision loss
  • Nausea, abdominal cramps, and vomiting
  • Weight loss
  • Extreme fatigue
  • Severe headaches
  • Coma


  • Laboratory tests
    Testing associated with HIV/AIDS involves detecting HIV antigen (p24) and/or the affected person's response to HIV (antibodies), measuring the amount of virus, or detecting the viral nucleic acid. The goals of HIV testing are to:

    • Screen for and diagnose HIV infection
    • Measure and monitor the amount of virus in the person's blood (the viral load)
    • Evaluate HIV's resistance to available drug therapies

    HIV testing is the only way for someone to know if he or she has HIV infection. Early detection and treatment of HIV infection and immune system monitoring can greatly improve long-term health. Also, if a person knows his or her HIV status, it may help change behaviors that can put that person and others at risk.

    Several organizations recommend routine screening for HIV. The Centers for Disease Control and Prevention (CDC), American College of Physicians (ACP), and the U.S. Preventive Services Task Force (USPSTF) recommend that anyone between the ages of 13 and 64 (or 15 to 65 in the case of the USPSTF) and pregnant women be screened for HIV at least once.

    Certain individuals should get tested at least once to learn their status, even if they are not between the ages of 13 and 64. These include:

    • People diagnosed with hepatitis, tuberculosis (TB) or a sexually transmitted disease
    • People who received a blood transfusion between 1978 and 1985 or had a sexual partner who received a transfusion and later tested positive for HIV
    • A healthcare worker with direct exposure to blood on the job
    • Any individual who thinks they may have been exposed

    Screening at least annually is advised for those at high risk for HIV and is recommended when an individual:

    • Has had unprotected sex with more than one partner since the last HIV test
    • Is a man who has had sex with another man (CDC suggests that gay or bisexual men may benefit from more frequent screening, such as every 3 to 6 months)
    • Has used street drugs by injection, especially when sharing needles and/or other equipment
    • Has exchanged sex for drugs or money
    • Has an HIV-positive sex partner
    • Has had sex with anyone who falls into one of the categories listed above or is uncertain about their sexual partner's risk behaviors

    For additional details on screening recommendations, see the articles for Teens, Young Adults, Adults, and Adults 50 and Up as well as Pregnancy and Newborns.

    Different types of tests may be used for HIV screening:

    • Combination HIV antibody and antigen test—this is the recommended screening test for HIV. It is available only as a blood test. It detects the HIV antigen called p24 plus antibodies to HIV-1 and HIV-2. (HIV-1 is the most common type found in the United States, while HIV-2 has a higher prevalence in parts of Africa.) The level of p24 antigen and the amount of virus (viral load) increase significantly soon after initial infection. Testing for p24 allows for detection of early infections, before HIV antibody is produced. About 2-8 weeks after exposure, antibodies to HIV are produced in response to the infection and remain detectable in the blood thereafter, making the antibody test useful for detecting infections weeks after exposure. By detecting both antibody and antigen, the combination test increases the likelihood that an infection is detected soon after exposure.
    • HIV antibody testing—all HIV antibody tests used in the U.S. detect HIV-1 and some tests have been developed that can also detect HIV-2. These tests are available as blood tests or tests of oral fluid.
    • p24 antigen testing—this is used alone without the antibody test only in rare cases when there is a question about interference with an HIV antibody test.

    There are a few different ways a person can get access to HIV screening:

    • A blood or oral sample can be collected in a health practitioner's office or a local clinic and sent to a laboratory for testing. Certain testing centers provide either anonymous (the name is never given) or confidential (the name is given but kept private) HIV testing and counseling. People can also contact their state, county, or city health department to find out where testing may be available. To find a testing site near you, visit the National HIV and STD Testing Resources.
    • In these same settings, there may be a rapid test available in which results are generated in about 20 minutes.
    • A home collection kit is available that allows a person to take a sample at home and then mail it to a testing center. Results are available over the phone, along with appropriate counseling.
    • There is a home test for HIV test that uses an oral sample and results are available in about 20 minutes. This allows the person tested to remain anonymous and to get confidential results. The home test has two limitations: 1) testing on oral fluid is less sensitive than a blood test so the home test may miss some cases of HIV that a blood test would detect; and 2) the home test is not as accurate when it is performed at home by a lay person compared to when it is performed by a trained health professional. However, the convenience of home testing might encourage some people who might otherwise be reluctant to go to a health practitioner or clinic to learn their HIV status.


    If any of the above screening tests is positive, then it must be followed by a second test to establish a diagnosis. This second test is an antibody test that is different than the first test. If the second test does not agree with the first test, then a third test is performed that detects the genetic material (RNA) of the virus.

    In 2014, the CDC updated its HIV testing recommendations to include a new testing protocol, which has been accepted by the Clinical Laboratory Standards Institute (CLSI):

    1. Screen for HIV using a combination HIV antigen/antibody test, then
    2. Verify a positive with a second HIV antibody test that differentiates between HIV-1 and HIV-2.
    3. If results between the first and second test do not agree, then the next test to perform is an HIV-1 RNA test (nucleic acid amplification test, NAAT). If the HIV-1 RNA is positive, then the test is considered positive.


    HIV Testing Algorithm
    HIV Screening Algorithm. Courtesy of James Faix, MD


    Two tests once commonly used to test for HIV, HIV-1 Western blot and HIV-1 immunofluorescence assay, are not included in this new protocol and should not be used since these tests detect antibody later in the infection (around 28 days) and may give a false-negative result.

    Other tests:

    • HIV viral load testing—measures the amount of HIV in the blood; it is performed when a person is first diagnosed to help determine the status of the disease and is ordered at intervals to monitor the effectiveness of therapy.
    • CD4 count—measures the number of CD4 T-cells in the blood; it is ordered when a person is first diagnosed to get a baseline assessment of the immune system and done at intervals to monitor therapy and the status of the immune system. If a person is doing well on treatment, this test may be done yearly.
    • HIV genotypic resistance testing—ordered when someone is initially diagnosed to determine whether the particular strain(s) of HIV that the person has is resistant to certain antiretroviral drug therapies; also ordered when treatment is changed or when there is evidence of treatment failure.
    • Phenotypic resistance testing–sometimes ordered for those who are resistant to multiple antiretroviral drugs to help guide treatment; this test evaluates whether the person's strain(s) of HIV can be inhibited by various concentrations of antiretroviral drugs.
    • Individuals who are planning to take the drug abacavir may be tested first for the gene allele, HLA-B*5701. If they are positive for it, they are at an increased risk of having a potentially severe hypersensitivity reaction and another drug should be considered. (Read more in the article on Pharmacogenetic Tests.)

    A number of other laboratory testing may be performed as part of overall care of an HIV-infected individual. Several tests may be done to identify and monitor the treatment of opportunistic infections, complications, and drug toxicities. Testing may also be ordered at intervals to evaluate the person's health and organ function. Some examples include:

    • Complete blood count
    • Comprehensive metabolic panel
    • Urinalysis
    • Tests for other sexually transmitted diseases, such gonorrhea or syphilis
    • Tests for other infections such as viral hepatitis or tuberculosis

    Non-laboratory tests
    Testing such as a chest X-ray or an imaging scan may sometimes be performed to help evaluate the person's health status.


    There is currently no cure for HIV infection and no vaccine to protect against it, but avoiding high-risk activities such as having unprotected sex and sharing needles for injecting drugs can help to prevent its spread. Routine screening for HIV has been recommended by the Centers for Disease Control and Prevention (CDC) and several other organizations to help identify HIV infections in people who may have no signs or symptoms. The early diagnosis of HIV infection is important to prevent its transmission to others and to allow evaluation, monitoring, and early treatment of the affected person.

    Treatment of HIV-infected mothers during pregnancy, precautions at birth, and avoiding breast-feeding can minimize the risk of passing the infection from mother to child. Giving the antiretroviral drug zidovudine intravenously during labor and delivery and also to the newborn twice a day by mouth for 6 weeks reduces the rate of transmission from 25-33% to about 1-2%. A combination of antiretroviral therapies is most effective at reducing the risk of HIV transmission to the baby.

    Healthcare workers can protect themselves from HIV infection by following universal precautions, such as wearing gloves and avoiding needle sticks.

    While there is no vaccine that prevents HIV, the CDC and the World Health Organization (WHO) recommend that individuals without HIV infection but at high risk for it consider taking pre-exposure prophylaxis (PrEP), a daily pill to help prevent infection. For people taking PrEP consistently, the risk of HIV infection was up to 92% lower compared to those who didn't take it.


    The goals of HIV and AIDS treatment are to suppress the virus to undetectable levels and to preserve the person's immune function and health. Suppressing viral replication prevents or inhibits HIV mutation and the development of drug resistance. It slows the progression of the disease and allows the number of CD4 T-cells to increase, improving immune function. Treatment of complications and opportunistic infections is also important, as is addressing drug therapy side effects and toxicity.

    The U.S. Department of Health and Human Services (DHHS) Panel on Antiretroviral Guidelines for Adults and Adolescents as well as WHO recommend that all individuals diagnosed with HIV receive treatment as soon as possible, including pregnant women. With advances in treatment, individuals with HIV are living longer, healthier lives. Once someone begins treatment, it is important that the person continue it for the rest of their life to help maintain health and prevent spread of HIV. Interruptions in treatment can lead to increases in the amount of virus (viral load) and can increase the risk of developing drug resistance, decrease immune function, and allow disease progression.

    Drug selection

    A person may be infected with drug-susceptible and/or drug-resistant strains of HIV. Testing for drug resistance is performed when a person is first diagnosed to help guide therapy. There are several classes of antiretroviral drugs used to treat HIV/AIDS. People typically take at least three drugs from two different classes in order to prevent or minimize virus replication and the emergence of drug-resistant strains. Combinations of three or more antiretroviral drugs are referred to as highly active antiretroviral therapy or HAART. There are preferred treatment regimens, but the specific drugs given must be tailored to the individual and to the strain(s) of HIV with which he or she is infected.

    Drug therapies may be evaluated and changed as necessary if the person experiences treatment failure, indicating the development of resistance to one or more of the drugs the person is taking. Another reason treatment may be changed is if the individual experiences significant side effects and toxicity. This may be related to the person's ability to absorb and metabolize the drug(s).

    People with HIV/AIDS will need to work closely with their healthcare provider(s) throughout their lifetime to adjust their medications to their changing needs. Treatment of people who have developed resistance to one or more drugs or classes of drugs can become challenging. Those affected may need to consult with health practitioners who specialize in the management of "treatment-experienced" patients. New HIV/AIDS drug treatments are continually being developed and brought into clinical use.

    For more details, read Overview of Treatments from AIDS.gov.


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