HIV stands for Human Immunodeficiency Virus.

It attacks the body's immune system, making it hard to fight off infections. It targets specific white blood cells known as CD4 cells. The lower a person's CD4 count, the weaker their immune system will be.

Symptoms will vary enormously across individuals. Many will become infected and experience nothing. Others weeks or months after exposure will have a flu like illness, develop an rash, have night sweats or more rarely have a life threatening brain infection. All too often people coming for a test report vague symptoms similar to the above but eventually test negative - is no evidence of infection. a blood test is therefore the only reliable means of detection.

For someone to become infected, a sufficient amount of HIV must get into their blood. Body fluids which contain enough HIV to infect someone are blood, semen, vaginal fluids including menstrual blood, and breast milk.

Saliva, sweat and urine do not contain enough virus to infect someone. HIV cannot pass through intact external skin, or through the air like a cold or flu virus.

The main ways in the UK of passing HIV on to another person are:

1. Sex without a condom - HIV can pass from one person to another through unprotected anal or vaginal sex. A small risk exists with oral sex.
2. Injecting drug use - HIV can be passed on by using needles or syringes and other injecting equipment (works) that someone with HIV has already used.
3. Mother to baby - a pregnant woman may pass the virus to her baby before or during birth, or HIV can be passed on during breastfeeding.
4. Organ transplant, blood transfusion or blood products - Since 1985, all blood and tissue donations and blood products in the UK have been screened for HIV.

The HIV antibody test is the most common method of checking for HIV . It looks for antibodies produced by the body in response to the presence of HIV. When someone becomes infected with HIV, it can take up to three months for their immune system to produce enough antibodies to show up on an HIV test. very rarely this takes longer - up to six months. The gap between infection and being able to test positive for HIV is known as the window period. Taking an HIV test less than three months after possibly getting infected might not give an accurate result.
However, throughout the window period , the infected person has enough virus in the blood, breast milk or sexual fluids to infect another person even though it wont show on a test.

A proper and consistent use of condoms is the most effective way of reducing risk in the sexually active person. Limiting the number of sexual partners may also help.
Avoiding injecting drugs or not sharing 'works' will prevent the virus from spreading.

There is no cure or vaccine for HIV. Some powerful anti HIV drugs are available. Taking these in various combinations can slow down the damage HIV does to the immune system.

AIDS is short for Acquired Immune Deficiency Syndrome. An HIV damaged immune system leaves a person open to other illnesses - ie tuberculosis, pneumonia and cancers. These would not normally be a threat. Once diagnosed with such an illness the person is often said to have AIDS. It is a term falling out of use. Instead it is increasingly called 'late stage' or 'advanced HIV infection'