HIV is a virus which attacks the immune system, and weakens your ability to fight infections and disease. It's most commonly caught by having sex without a condom.

It can also be passed on by sharing infected needles and other injecting equipment, and from an HIV-positive mother to her child during pregnancy, birth and breastfeeding.

HIV stands for human immunodeficiency virus. The virus attacks the immune system, and weakens your ability to fight infections and disease.

There is no cure for HIV, but there are treatments to enable most people with the virus to live a long and healthy life.

AIDS is the final stage of HIV infection, when your body can no longer fight life-threatening infections. With early diagnosis and effective treatment, most people with HIV will not go on to develop AIDS.

How do you get HIV?

HIV is found in the body fluids of an infected person, which includes semen, vaginal and anal fluids, blood, and breast milk. It is a fragile virus and does not survive outside the body for long. 

HIV cannot be transmitted through sweat or urine.

The most common way of getting HIV in the UK is by anal or vaginal sex without a condom. According to statistics from Public Health England, 95% of those diagnosed with HIV in the UK in 2013 acquired HIV as a result of sexual contact.

Other ways of getting HIV include:

• using a contaminated needle, syringe or other injecting equipment
• tranmission from mother to baby during pregnancy, birth or breastfeeding
• through oral sex or sharing sex toys (although the risk is significantly lower than for anal and vaginal sex) 

Read more about what causes HIV.

Getting tested

The only way to find out if you have HIV is to have an HIV test.

If you think you have put yourself at risk of HIV, you should seek medical advice and have a test as soon as recommended. The earlier HIV is diagnosed, the earlier you can start treatment and avoid becoming ill.

Emergency anti-HIV medication called PEP (post-exposure prophylaxis) may stop you becoming infected, but treatment must be started within three days of coming into contact with the virus.

There are a number of places you can get an HIV test, including your GP surgery and sexual health clinics and clinics run by charities including the Terrence Higgins Trust.

Most HIV tests in the UK involve taking a small sample of blood and sending this to a laboratory for analysis. These tests can provide a reliable result from four weeks after possible infection and results are usually available within a few days. It is also possible to test using a saliva sample or pin-prick (blood-spot) test, and many sexual health clinics now use these tests routinely as the result is available within a few minutes and do not need to be sent to the lab. However, these tests do not reliably detect HIV if you have been infected within the past few weeks.

You may get the results in hours, days or weeks, depending on the type of test you take.

If your test is positive, you will be referred to a specialist HIV clinic where you'll have more blood tests to show what effect HIV is having on your immune system and be able to discuss treatment options.

Find out more about coping with a positive HIV test.

Anyone who has sex without a condom or shares needles is at risk of HIV infection. However, the two groups with highest rates of HIV in the UK are gay and bisexual men and African men and women.

NICE recommends that annual HIV tests be offered to all men who have sex with men, and more frequent testing be offered to those at higher risk due to multiple partners or unsafe sexual practices.

Living with HIV

Although there is no cure for HIV, treatments are now very effective, enabling people with HIV to live long and healthy lives.

Medication, known as antiretrovirals, work by stopping the virus replicating in the body, allowing the immune system to repair itself and preventing further damage. These medicines come in the form of tablets, which need to be taken every day.

HIV is able to develop resistance to a single HIV drug very easily, but taking a combination of different drugs makes this much less likely. Most people with HIV take a combination of three antiretrovirals and it is vital that the medications are taken every day as recommended by your doctor.

You will be encouraged to take regular exercise, eat a healthy diet, stop smoking and have yearly flu jabs and five-yearly pneumococcal vaccinations to minimise the risk of getting serious illnesses.

Without treatment, the immune system will become severely damaged and life-threatening illnesses such as cancer and severe infections can occur. This is known as late-stage HIV infection or AIDS.

Read more about living with HIV.

Preventing HIV

Anyone who has sex without a condom or shares needles is at risk of HIV infection.

The best way to prevent HIV is to use a condom for sex and to never share needles or other injecting equipment (including syringes, spoons and swabs). Knowing your HIV status and that of your partner is also important.

How common is HIV?

At the end of 2013, there were an estimated 107,800 people in the UK living with HIV. The majority were infected through sex (43,500 gay and bisexual men and 59,500 heterosexuals).

A quarter of people with HIV (over 26,100) do not know they are infected.

Around one in every 360 people in the UK has HIV, but the two groups with highest rates of HIV are gay and bisexual men and Black African heterosexuals, where the rates are approximately one in 17 and one in 18 respectively.

The World Health Organization estimates that around 35 million people in the world are living with HIV.

The virus is more common in sub-Saharan African countries, such as South Africa, Zimbabwe and Mozambique.

Find out more about:

• the symptoms of HIV
• what causes HIV
• treating HIV
• visiting an STI clinic