Any infection that begins suddenly, with intense or severe symptoms, is called acute (or primary). If the illness lasts more than a couple of weeks, it is called chronic.

Adherence measures how faithfully a person takes all antiretroviral medications at the right time. Poor adherence is one of the main reasons antiretroviral combinations fail.

AIDS stands for acquired immunodeficiency syndrome and is a combination of diseases that don’t normally appear in people with healthy immune systems. It is the final stage of HIV infection in which the body’s immune system is unable to fight off infections and other illnesses that take advantage of a weakened immune system. A person infected with HIV is diagnosed with AIDS when he or she has one or more of these conditions and/or if he or she has a dangerously low number of CD4 cells (less than 200 cells per cubic millimetre (mm3) of blood).

An immune system protein formed in response to invading disease agents such as viruses, fungi, bacteria, and parasites. Usually antibodies defend the body against invading disease agents, however, in most cases of HIV infection, the antibody response is not able to control the infection.

An invading substance that may be the target of antibodies.

A treatment that may prevent HIV from further damaging the immune system by blocking or hampering the reproduction of the HIV virus.

A substance that stops or suppresses the reproduction of a virus.

AIDS Therapy Evaluation in the Netherlands project (ATHENA). Stichting HIV Monitoring was founded in 2001 as a result of the successful ATHENA project.

An initial measurement used as the basis for future comparison. For people infected with HIV, baseline testing includes CD4 count, viral load (HIV RNA), and resistance testing. Baseline test results are used to guide HIV treatment choices and monitor effectiveness of antiretroviral therapy (ART).

Combination antiretroviral treatment . Available since 1996, this treatment for HIV consists of a combination of two or more antiretroviral drugs.

CD4+ T-lymphocyte, or T4-cell or T-helper cell. A white blood cell that plays a vital role within the immune system and can be infected by the HIV virus. In the course of the HIV infection the number of CD4 cells may drop from normal levels (+ 500 per mm³) to dangerously low levels (fewer than 200 CD4 cells per mm3 of blood).

US Centers for Disease Control and Prevention.

Centre for Infectious Disease Control Netherlands, National Institute for Public Health and Environment (www.rivm.nl/cib).

When a person has two or more infections at the same time. For example, a person infected with HIV may be co-infected with hepatitis C (HCV) or tuberculosis (TB) or both.

When a person has two or more diseases or conditions at the same time. For example, a person with high blood pressure may also have heart disease.

After a person becomes resistant to one particular drug, they may develop resistance to similar drugs, without ever having been exposed to these drugs. This is known as cross-resistance.

Direct-acting antivirals (DAAs) are new-generation drugs that treat hepatitis C by targeting specific steps in the hepatitis C virus life cycle. There are different classes of DAAs, defined by their mechanism of action and therapeutic target. 

Deoxyribonucleic acid. A complex protein that carries genetic information. HIV can insert its own genetic material into the DNA molecules of human CD4 cells and either establish dormant infection or replicate and form new virus particles.

The study of the distribution, causes, and clinical characteristics of disease or health status in a population.

The genotype is the underlying genetic makeup of an organism.

Dutch public health service (www.ggd.nl).

The time it takes a drug to lose half its original concentration or activity after being introduced into the body. Drug half-life is considered when determining drug dosing.

Pertaining to the liver.

A viral infection that affects the liver and is transmitted only through blood-to-blood and sexual contact.

A viral infection that is transmitted primarily by blood and blood products, as in blood transfusions or intravenous drug use, and sometimes through sexual contact.

Human Immunodeficiency Virus; the virus that causes the Acquired Immunodeficiency Syndrome (AIDS). HIV attacks and destroys the immune system by entering and destroying the cells that control and support the immune response system.

The HIV type responsible for the majority of HIV infections worldwide.

A virus very similar to HIV-1 that has been found to cause immune suppression. HIV-2 infections are found primarily in Africa.

Dutch HIV patients’ association (https://www.hivvereniging.nl/)

If treatment is effective and HIV is well-controlled, the immune cells regain their normal function and CD4 cell counts are close to normal. This is defined as immune recovery.

A type of HIV treatment failure. There is no consensus on the definition of immunological failure. However, some experts define it as the failure to achieve and maintain adequate CD4 counts despite viral suppression.

Interferons are naturally-occurring proteins (cytokines) produced by immune cells in response to an antigen, usually a virus. Although they don’t directly kill viral cells, they boost the immune response by signalling neighbouring cells into action and inhibiting the growth of malignant cells. There are three types of interferons: alpha, beta, and gamma. Laboratory-made interferons are used to treat certain cancers and opportunistic infections. Addition of polyethylene glycol to interferons prolongs the half-life of interferon. Pegylated interferon alpha is used to treat chronic hepatitis C infection.

When a person has only one infection.

Mortality rate is a measure of the frequency of occurrence of death among a defined population during a specified time period.

Men who have sex with men.

Netherlands Federation of University Medical Centres.

Diseases and clinical events that are not related to AIDS (i.e. that are not listed as being associated with AIDS by the Centers for Disease Control and Prevention) and include conditions such as malignancies, end-stage renal disease, liver failure, pancreatitis, cardiovascular disease.

Antiretroviral (ARV) HIV drug class. Non-nucleoside reverse transcriptase inhibitors (NNRTIs) bind to and block HIV reverse transcriptase (an HIV enzyme). HIV uses reverse transcriptase to convert its RNA into DNA (reverse transcription). Blocking reverse transcriptase and reverse transcription prevents HIV from replicating.

Antiretroviral (ARV) HIV drug class. Nucleoside reverse transcriptase inhibitors (NRTIs) block reverse transcriptase (an HIV enzyme). HIV uses reverse transcriptase to convert its RNA into DNA (reverse transcription). Blocking reverse transcriptase and reverse transcription prevents HIV from replicating.

A building block of nucleic acids. DNA and RNA are nucleic acids.

A type of antiretroviral (ARV) HIV drug. Nucleotide reverse transcriptase inhibitors interfere with the HIV life cycle in the same way as NRTIs. Both block reverse transcription and therefore nucleotide reverse transcriptase inhibitors are included in the NRTI drug class.

Dutch Association of HIV-Treating Physicians (Nederlandse Vereniging van HIV Behandelaren)

A measure of time used in medical studies that combines the number of persons and their time contribution (e.g., in years) to the study. In the ATHENA cohort, person years generally refers to the cumulative number of years that individuals were followed by Stichting HIV Monitoring.

Perinatal transmission of HIV refers to the passage of HIV from an infected mother to her child during pregnancy, labour and delivery, or breastfeeding (through breast milk).

A type of enzyme that breaks down proteins into smaller proteins or smaller protein units, such as peptides or amino acids. HIV protease cuts up large precursor proteins into smaller proteins. These smaller proteins combine with HIV’s genetic material to form a new HIV virus. Protease inhibitors (PIs) prevent HIV from replicating by blocking protease.

Antiretroviral (ARV) HIV drug class. Protease inhibitors (PIs) block protease (an HIV enzyme). This prevents new HIV from forming.

A class of viruses which includes HIV. Retroviruses are so named because they carry their genetic information in RNA rather than DNA, and the RNA information must be translated "backwards" into DNA.

After infecting a cell, HIV uses an enzyme called reverse transcriptase to convert its RNA into DNA and then replicates itself using the cell’s machinery.

A type of nucleoside inhibitor prescribed for the treatment of hepatitis C in combination with an interferon. Ribavirin stops the hepatitis C virus from spreading by interfering with the synthesis of viral RNA.

The Dutch National Institute for Public Health and the Environment (www.rivm.nl).

The change from an absence of HIV antibodies in the blood to the presence of those antibodies.

Stichting HIV Monitoring (the Dutch HIV monitoring foundation, www.hiv-monitoring.nl).

Undetectable hepatitis C virus (HCV) in blood 12 or 24 weeks after completion of antiviral therapy for chronic HCV infection.

The continuous, long-term suppression of a person’s viral load (HIV RNA), generally to undetectable levels, as the result of treatment with antiretroviral drugs. Viral suppression does not mean a person is cured; HIV still remains in the body.

The extent to which a drug’s side effects can be tolerated by an inidividual.

The presence of a virus in the blood.

A type of HIV treatment failure. Virological failure occurs when antiretroviral therapy (ART) fails to suppress and sustain a person’s viral load to below 200 RNA copies/mL. Factors that can contribute to virologic failure include drug resistance, drug toxicity, and poor treatment adherence.

The number of HIV particles in a millilitre of blood or another body fluid, such as semen or cerebrospinal fluid.

Dutch Ministry of Health, Welfare and Sport (www.rijksoverheid.nl).

Some of the above definitions were taken from www.aidsinfo.nih.gov/ (with permission)