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HIV is short for 'human immunodeficiency virus'. If you become infected with HIV, you slowly lose the ability to defend yourself against other infections and certain types of cancer.

HIV destroys cells that are part of your immune system, the name given to the parts of your body that protect you from infections. This reduces your ability to fight off certain other infections. When this happens, you have AIDS. AIDS stands for 'acquired immunodeficiency syndrome'.

You may be devastated to find out you have HIV infection. But having HIV isn't as bad news as it used to be. Treatments can't get rid of HIV completely, but they can keep HIV and other infections under control.

• The only way to find out whether you have HIV is to have a blood test.
• HIV lives in blood and in men's semen and women's vaginal fluid. You can get the virus when fluids from an infected person get inside your body.
• Most people get HIV by having unprotected sex with someone who has the virus.
• You can also get the virus by sharing a needle or a syringe with someone who injects drugs.
• Pregnant women with HIV can pass the virus to their baby during pregnancy, birth or when breastfeeding.¹
• You can't get HIV from normal non-sexual contact such as kissing on the cheek or sharing a fork or a bar of soap with someone who has the virus.¹
• You can have HIV for many years before it starts to affect your health.¹
• There's no cure for HIV. But there are good treatments that can help you keep the virus under control.

Your immune system is the name given to the parts of your body that protect you against infections. Bacteria, viruses, fungus and parasites are all types of germs that may make you sick. If you get infected, your immune system tries to kill the bug.

Your immune system includes many different types of immune cells. They travel in your blood looking for infections. When your immune cells meet a virus, for example, they recognize that the virus may be dangerous. They work together to try to destroy the virus. Your immune system also kills any of your own cells that have begun to grow incorrectly. For example, it can help stop cancers from developing.

Certain cells called CD4 cells (or T helper cells) help pass a message to other cells to help destroy the virus. It can take many days for your immune system to get rid of a virus.

HIV is a type of virus called a retrovirus. When it meets your immune cells it enters and destroys them. So, your immune system can't fight back and get rid of HIV, like it does with other viruses.

• When HIV gets into your body, the virus gets inside your CD4 cells, where it can live for many years.
• At some point, HIV starts to make thousands of copies of itself. These copies of HIV leave the CD4 cell, killing it at the same time.
• These copies of HIV move on to other CD4 cells, eventually killing them.
• Gradually, the number of CD4 cells in your blood (called your CD4 count) drops very low.
• With fewer CD4 cells in your blood, you start to get infections and types of cancer that you wouldn't normally get. When this happens, doctors say you have AIDS.
• Even without treatment you may not get extra infections for 10 years after you were infected, and that's without treatment to slow down the virus.² If you take treatments, it may be 20 years or more before you start getting extra infections. We don't know exactly how long treatments can protect you, because the new combinations have not been used for very long. It will also depend on how healthy you are when you start treatment.

To learn more, see How HIV damages your immune system.

HIV doesn't spread through the air like cold and flu viruses. So you can't catch it by being next to someone who has HIV and breathing in after they have sneezed.

HIV lives in people's blood and in some other body fluids. To get HIV, one of these fluids from someone with HIV has to get into your blood.³ The chance of catching HIV is different with different fluids and with different activities. For example, it is easier to become infected with HIV from anal sex than through vaginal sex.

Blood contains the most virus particles. You only need a tiny amount of blood from someone with HIV to get into your body to become infected.

In men, HIV also lives in semen. And it may be possible to get HIV from the fluid that comes out of the penis before ejaculation.³

In women, HIV lives in vaginal fluid. And in women who have recently had a baby, the virus can get into breast milk.

It's important to remember that you can't get HIV from saliva, tears, sweat, feces or urine.³

• HIV can get into your blood through cuts or sores on your skin.
• HIV can get through the thin, moist lining of your anus, rectum or genitals. These thin layers are called mucous membranes. HIV can also enter your body through the lining of your mouth and eyes.
• HIV can't get through normal skin that's not broken. So you won't get infected if some blood from a person with HIV splashes on your unbroken skin.

Here are the main ways that people get infected with HIV:³

• Through having unprotected sex. This is the most common way that people get infected with HIV. For more information, see HIV and unprotected sex
• From sharing needles or syringes. For more information, see HIV and sharing needles
• Because of HIV passing from mother to baby. For more information, see HIV passing from mother to baby
• From accidentally sticking themselves with an infected needle or being splashed with blood in a laboratory accident. These are very rare ways to get infected with HIV. For more information, see HIV and accidentally sticking yourself with a needle
• Through a blood transfusion. This is also very rare these days. For more information, see HIV and blood transfusions.

There are lots of myths about how you can get HIV. You may have many questions about how you can and can't get HIV. For example, some people worry they can get it by kissing. To learn more, see Your questions answered.

You're at risk of getting HIV if:

• You have unprotected sex with someone who is infected. Unprotected means not using a condom.
• You share someone else's needle or syringe to inject drugs
• Your mother had the virus when you were born
• You're a health care worker (because you may have an accident with a needle).

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.