In the UK, most cases of HIV are caused by having sex with a person who has HIV without using a condom. A person with HIV can pass the virus on to others even if they do not have any symptoms. People with HIV can pass the virus on more easily in the weeks following infection. HIV treatment significantly reduces the risk of someone with HIV passing it on.

Sexual contact

Most people diagnosed with HIV in the UK acquire the virus through unprotected vaginal or anal sex.

Who's most at risk?

People who are at higher risk of becoming infected with HIV include:

• people with a current or previous partner with HIV
• people with a current or previous partner who is from an area with high HIV rates
• people who are from an area with high HIV rates
• people who engage in chemsex (using drugs to help or enhance sex)
• men who have unprotected sex with men
• women who have unprotected sex with men who have sex with men
• people who inject drugs and share equipment
• people who have unprotected sex with somebody who has injected drugs and shared equipment
• people with a history of sexually transmitted infections, hepatitis B or hepatitis C
• people who have had multiple sexual partners
• healthcare workers who could accidentally prick themselves with an infected needle – but this risk is extremely low

Most people infected with HIV experience a short, flu-like illness that occurs 2-6 weeks after infection. After this, HIV may not cause any symptoms for several years. It's estimated up to 80% of people who are infected with HIV experience this flu-like illness. The most common symptoms are:

• raised temperature (fever)
• sore throat
• body rash

Other symptoms can include:

• tiredness
• joint pain
• muscle pain
• swollen glands

The symptoms usually last 1-2 weeks, but can be longer. They're a sign that your immune system is putting up a fight against the virus.  But having these symptoms does not necessarily mean you have the HIV virus. Remember: they're commonly caused by conditions other than HIV.

If you have several of these symptoms and think you've been at risk of HIV infection within the past few weeks, you should get an HIV test. After the initial symptoms disappear, HIV may not cause any further symptoms for many years.

During this time, the virus continues to be active and causes progressive damage to your immune system. This process can vary from person to person, but may take up to 10 years, during which you'll feel and appear well. Once the immune system becomes severely damaged, symptoms can include:

• weight loss
• chronic diarrhoea
• night sweats
• skin problems
• recurrent infections
• serious life-threatening illnesses

Earlier diagnosis and treatment of HIV can prevent these problems.