Human immunodeficiency virus (HIV) - facts

HIV belongs to a group of viruses called retroviruses. It is also a lentivirus (lente = slow), which means that it takes a long time from infection until the disease progresses.

​As with all other viruses, HIV also depends on living cells to reproduce. HIV attacks certain immune cells in the body and uses these as a factory to make millions of new viruses which can then infect new, healthy cells. The cells that are attacked are destroyed in the process. The destruction of the cells will gradually weaken the immune system because they belong to the most important group in the immune system.

How is HIV transmitted?

​HIV is found in the bodily fluids of an infected person. The fluids that contain enough viruses to transmit infection are blood, semen, vaginal secretions and breast milk. Other bodily fluids (sweat, tears, saliva and urine) contain negligible amounts of virus and involve virtually no risk of infection.

Sexual contact

Sexual contact is the most common mode of transmission. Over 90 % of HIV infections occur by sexual contact. Worldwide, heterosexual infection is most common. The virus is transmitted when blood, semen or vaginal secretions come into contact with the mucous membranes of another person. Mucous membranes are found in the vagina, urethra, rectum, mouth and eyes.

Transmission by sexual contact may therefore occur during vaginal / anal intercourse or oral sex. Anal intercourse is considered to be the most risky, but it is possible to be infected with HIV by oral sex. HIV is transmitted more easily from man to woman than from woman to man. Among men who have sex with men, the man who receives the semen is at greatest risk of infection.

Other sexually transmitted diseases increase the risk of contracting HIV through sexual contact.  Sexually transmitted diseases such as

  • chlamydia
  • gonorrhoea
  • herpes
  • syphilis or other infections with genital sores

Pre-existing vaginal infections caused by yeast also increase the risk of infection. The increased risk of contracting HIV applies whether the person with the sexually transmitted infection / vaginal infection either has HIV infection or is exposed to the risk of HIV infection.

Transmission via blood

HIV is found in blood. It can be transmitted by blood transfusion, a risk that is now more or less eliminated in Norway because of blood donor testing and heat treatment of blood products. HIV can also be transmitted when significant amounts of blood come into contact with mucous membranes or damaged skin.

HIV can spread among injecting substance users when sharing syringes, needles or other equipment. This applies both for drugs used for doping and narcotic substances. Never share syringes or needles.

From mother to child

The risk of transmission from an untreated mother to child during pregnancy, birth or breastfeeding is approximately 30 %. This risk can be reduced to under 2 % if the mother receives treatment during pregnancy, the child receives antiviral treatment in the first weeks after birth, and the child is not breastfed.

The immune system

​The immune system is designed to protect us against a range of diseases. It consists of white blood cells that are made in the bone marrow. We have a number of different white blood cells with their own specific task. A cell type called the T lymphocyte is particularly important. White blood cells recognise foreign bodies and neutralise or remove them. This applies especially to microbes, whether they are bacteria, virus, fungi or other infectious agents. The immune system is also important in protecting us against cancer.

How does HIV destroy the immune system?

HIV will only attack and reproduce in one cell type. The virus needs to bind to a specific molecule on the cell surface, known as CD4, to access the cell. There are many cell types in the body with CD4 molecules on the surface that can be attacked by HIV. The most important white blood cells are called T4 lymphocytes or helper cells. They play a key role in the immune response because they direct all the other cells and signals.

Healthy people have between 800 and 1,200 CD4 lymphocytes per microlitre of blood. When HIV attacks a CD4 cell and uses it to produce new viruses, the cell is destroyed. HIV destroys a large number of CD4 cells every day and eventually the bone marrow will be unable to maintain the production of new cells. The number of CD4 cells will therefore be gradually reduced. When the CD4 count sinks to 200-300 per microlitre, the immune system is so impaired that the person can contract illnesses that the body would otherwise be able to control.

What are opportunistic infections (OI)?

When the immune system is significantly weakened, the microbes that usually do not cause disease in people will attack. This is called opportunistic infection. Most opportunistic infections are caused by microbes that we carry after infection in early childhood. A normal immune system protects against these microbes. This protection disappears when the immune response fails, and the person becomes ill.

Infections caused by these microbes can often be kept under control with medication, but may reappear if the immune system is still compromised.

What is AIDS?

​AIDS is an abbreviation for Acquired Immunodeficiency Syndrome. Most countries have a well-defined combination of diseases and symptoms that, together with an HIV infection, are essential to diagnose AIDS. The definition varies from country to country, but this is often due to social security schemes rather than disagreements about the definition.

A number of countries provide better financial support if a person has an AIDS diagnosis than an HIV diagnosis. This is not the case in Norway. The most common reason for an AIDS diagnosis is an opportunistic infection. However, the first sign of AIDS in a person with HIV infection can be:

  • prolonged fever or diarrhoea
  • significant and unexplained weight loss
  • certain forms of cancer

Development of the HIV infection

Primary HIV infection

50-70 % of the HIV infected develop symptoms 2-4 weeks after infection. This is known as acute or primary HIV infection. The usual symptoms are

  • fever
  • sore throat
  • swollen lymph nodes
  • rash
  • muscle and joint pain

The symptoms may resemble influenza and mononucleosis (“glandular fever”) and usually last 2-3 weeks. After this phase, the symptoms disappear and the person feels healthy.

It is important to detect a primary HIV infection, because:

  • with primary HIV infection and the first months after infection, there is a high concentration of virus in the blood and therefore a high risk of infection.
  • with primary HIV infection, it is a short time since infection. This makes contact tracing easier and reduces the risk of further spread.
  • a specialist in infectious diseases should consider antiviral treatment in each case.

Even without treatment, 50 % of people with HIV infection will show no signs of disease after 10 years. People with HIV infection are at greater risk of tuberculosis infection progressing to disease


The amount of virus (viral load) in the blood after infection will gradually subside within 4-6 months and then stabilise at a certain level that varies from person to person. The disease progresses faster in people with a high stable viral load than with a lower viral load. The speed of disease progression varies from person to person. Statistics show that even without treatment, 50 % of people with HIV infection will have no symptoms of disease after 10 years.

Current treatment can effectively delay the deterioration of the immune system and the resulting complications. Therefore, these complications arise far less frequently now than in the days before effective treatment was available.

Early symptomatic HIV infection

In people with weakened immune systems, common diseases will occur more frequently and have a longer and more complicated course than in people with a normal immune system. When the number of CD4 cells decreases, other conditions could arise, such as fungal infections (thrush) in the mouth or the vagina. Other fungal infections such as athlete’s foot and nail fungus can be troublesome.

Herpes can cause serious problems, as can common warts and genital warts. Another early symptom of a weakened immune system can be shingles (herpes zoster). This is caused by a reactivation from childhood of the chickenpox virus. Skin problems are common, especially eczema with redness and flaking of the face and upper chest and back. Other skin diseases may also appear.

People with HIV infection have a greater risk that a tuberculosis infection will progress to disease, and reactivation of tuberculosis can be seen quite early in the course of an HIV infection. In Norway, few people have both HIV infection and tuberculosis but this combination is far more common globally. Tuberculosis should be considered for anyone with HIV infection, especially for those coming from countries with a high prevalence of tuberculosis.

Advanced HIV infection

Advanced HIV infection is seen less frequently today since most HIV infections are detected relatively early and treatment can begin before they develop. With CD4 counts of around 200, serious opportunistic infections may arise. These are infections by microbes that a healthy immune system would manage to control. Pneumonia caused by the fungus Pneumocystis jiroveci is serious if the diagnosis is delayed or proper treatment is not given.

Today, this form of pneumonia is mainly seen in people who are not being treated for HIV because they are not aware that they are infected.

Approximately 15 % of the population are carriers of the toxoplasmosis parasite, which can cause brain infection among people with weakened immune systems. Those who are not carriers and have a weakened immune system are advised to avoid contact with cats and to avoid meat that is not thoroughly cooked. Some viruses that we carry can be reactivated and cause disease.

Cytomegalovirus (CMV) is a virus that infects many, and approximately 60% of the adult population are carriers. With a weakened immune system, this virus may be reactivated and cause severe infection in a variety of body organs. The eyes are often affected. At the advanced stage of HIV infection, increased rates of cancer are also seen, including Kaposi’s sarcoma and lymphoma (cancer of the lymph nodes).

Other HIV-related diseases

Studies in recent years have shown that HIV positive people have an increased incidence of a variety of diseases that usually affect older, uninfected people. These include cardiovascular diseases, osteoporosis, liver diseases, kidney diseases, cancer and diabetes. This may be because HIV causes inflammation, particularly in the arterial walls, and an increased tendency to blood clots.

HIV positive people with other risk factors for disease (e.g., cardiovascular disease), together with side effects from HIV treatment may also affect the development of such diseases at an early age. As more HIV positive people age, it is important to check for other health problems than the traditional HIV-related diseases.

More information about HIV infection

On Norwegian Institute of Public Healths website you will find this information as brochures and videos in 13 other languages. Visit them here.

​Folkehelseinstituttet. Hivinfeksjon/Aids - veileder for helsepersonell [HIV/Aids - a guide for health personnel]. Oslo: 2019 [hentet 2019-02-25]. Tilgjengelig fra:

Norsk forening for infeksjonsmedisin. Faglige retningslinjer for oppfølging og behandling av hiv [Academic guidelines for follow-up and treatment of HIV]. Oslo: 2018 [hentet 2019-02-25]. Tilgjengelig fra:

Content provided by Norwegian Institute of Public Health

Last updated Tuesday, January 29, 2019