HIV and Viruses

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HIV is the Virus that Causes AIDS

HIV (Human Immunodeficiency Virus) is a virus that affects the immune system. It eventually leads to acquired immune deficiency syndrome (AIDS).

AIDS is a conditions where the immune system deteriorates and eventually fails.This makes someone with AIDS more vulnerable to other infections, like pneumonia.

HIV infects (and eventually kills) helper T-cells, which act as the host cells for the virus. Without enough helper T-cells, the immune system is unable to mount an effective response to infections because other immune system cells don't behave how they should as they don't receive the chemical signals from the helper T-cells, e.g. no phagocytes activates, no cytotoxic T-cells or B-cells activated.

People infected with HIV develop AIDS when the helper T-cell numbers in their bodies reach a critically low level.

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HIV has a Spherical Structure

A core that contains the generic material (RNA) and some proteins (including the enzyme reverse transcriptase, which is needed for virus replication).

An outer coating of protein called a capsid.

An extra outer layer called an envelope. This is made of membrane stolen from the cell membrane of a previous host cell.

Sticking out from the envelope are lots of copies of an attachment proteins that help HIV attach to the host helper T-cell. These proteins can also be called envelope proteins.

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HIV Replicates Inside its Host's Helper T-Cells

HIV (and all other viruses) can only reproduce inside the cells of the organism it has infected. HIV replicates indie the helper T-cells of the host. It doesn't have the equipment (such as enzymes and ribosomes) to replicate on its own, so it uses those of the host cell. Here's how HIV replicates:

The attachment protein attaches to a receptor molecule on the cell membrane of the host helper T-cell.

The capsid is released into the cell, where it uncoats and releases the genetic material (RNA) into the cell's cytoplasm.

Inside the cell, reverse transcriptase is used to make a complementary strand of DNA from the viral RNA template.

From this, double-stranded DNA is made and inserted into the human DNA.

Host cell enzymes are used to make viral proteins from the viral DNA found with the human DNA.

The viral proteins are assembled into new viruses, which bud from the cell and go on to infect other cells.

During the infection period, HIV replicates rapidly and the infected person may experience sever flu-like symptoms. After this period, HIV replication drops to a lower level. This is the latency period. During the latency period (which can last for years), the infected person won't experience any symptoms.

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People with AIDS are Susceptible to a Range of Illnesses

People with HIV are classed as having AIDS when symptoms of their failing immune system start to appear or their helper T-cell count drops below a certain level. People with AIDS generally develop diseases that wouldn't cause serious problems in people with a healthy immune system. The length of time between infection with HIV and the development of AIDS caries between individuals, but without treatment, it's usually around 10 years.

The initial symptoms of AIDS include minor infections of mucous membranes (e.g. the inside of the nose, ears and genitals) and recurring respiratory infections.

As AIDS progresses the number of immune system cells decreases further. Patients become susceptible to more serious infections including chronic diarrhoea, severe bacterial infections and tuberculosis.

During the late stages of AIDS patients have a very low number of immune system cells and can develop a range of serious infections such as toxoplasmosis of the brain (a parasite infections) and candidiasis of the respiratory system (fungal infections). It's these serious infections that kill AIDS patients, not HIV itself.

The length of time that people survive with AIDS varies a lot. Factors that affect progression of HIV to AIDS and survival time with AIDS include existing infections, the strain of HIV they're infected with, age and access to healthcare.

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Antibiotics Don't Work Against Viruses

Antibiotics kill bacteria by interfering with their metabolic reactions. They target the bacterial enzymes and ribosomes used in these reactions.

Bacterial enzymes and ribosomes are different from human enzymes and ribosomes. Antibiotics are designed to only target the bacterial ones so they don't damage human cells.

Viruses don't have their own enzymes and ribosomes - they use the ones in the host cell. So because human viruses use human enzymes and ribosomes to replicate, antibiotics can't inhibit them because they don't target human processes.

Most antiviral drugs are designed to target the few virus-specific enzymes that exist. For example, HIV uses reverse transcriptase to replicate. Human cells don't use this enzyme so drugs can be designed to inhibit it without affecting the host cell. These drugs are called reverse-transcriptase inhibitors.

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There's No Cure for HIV

There's currently no cure or vaccine for HIV but antiviral drugs can be used to slow down the progression of HIV infection and AIDS in an infected person.

The best way to control HIV infection in a population is by reducing its spread. HIV can be spread via unprotected sexual intercourse, through bodily fluids (e.g. blood from sharing contaminated needles) and from a HIV-positive mother to her fetus. Not all babies from HIV-positive mothers are born infected with HIV and taking antiviral drugs during pregnancy can reduce the chance of the baby being HIV-positive.