A record number of people in the UK are living with HIV, with the number of people with the virus reaching nearly 100,000, new figures show.
The Health Protection Agency (HPA) said there were about 96,000 people who have the virus - an all-time high.
But health officials warned a quarter of people who have the human immunodeficiency virus are not aware they have been infected.
There were 6,280 people diagnosed with HIV in 2011, the HPA said.
Nearly half of all diagnoses in 2011 were acquired heterosexually. Of these, more than half were probably acquired in the UK, compared to only 27% in 2002 according to the HPA report.
New diagnoses among gay men reached an all-time high in 2011, with 3,010 men discovering they were infected.
The HPA said that one in 20 men who have sex with men in the UK now have HIV, the figure soars to nearly one in 12 in London.
Black African people are also at a higher risk, the HPA said, with 37 per 1,000 living with the infection - overall HIV prevalence in the UK was 1.5 per 1,000 people.
Dr Valerie Delpech, HPA head of HIV surveillance, said: "These figures are a reminder of how vital safe sex programmes remain.
"Promoting HIV testing and condom use is crucial to tackling the high rates of transmission, late diagnosis and undiagnosed HIV still seen in the UK.
"National HIV Testing Week is a great opportunity to encourage people to get tested.
"We also encourage clinicians to take every opportunity to offer the test to those in higher risk groups and, in high prevalence areas, to all general medical admission and new GP registrants.
"The good news is that with the excellent services and treatments available nowadays, if diagnosed and treated early someone with HIV can look forward to a normal lifespan, as well as protecting their sexual partners from infection.
"That's why it is vitally important that anyone who has been at risk gets an HIV test, and that those in higher risk groups get screened regularly."
At risk groups include gay and bisexual men and African communities.
The Terrence Higgins Trust said regular testing can halt the spread of HIV.
Sir Nick Partridge, chief executive of the charity, said: "HIV is an entirely preventable condition, yet each year we see thousands more people across the UK receive this life-changing diagnosis.
"While there is still no cure and no vaccine, that doesn't mean we need to accept its continuing march.
"Reducing undiagnosed HIV by encouraging those in high-risk groups to test more regularly is one way we can put the brakes on the spread of infection.
"A simple HIV test, offered free at clinics and testing services all over the country, might add over 40 years to the life of someone with HIV, diagnosed in good time.
"Knowing your status also helps you protect your partners in the future.
"Thirty years on from the start of the epidemic, public understanding of HIV has dropped to a worrying level.
"As a result, we are starting to see a significant increase in the number of heterosexuals acquiring the virus in the UK.
"It is important that everyone, no matter their age or background, understands that nobody is immune from infection.
"We all have a responsibility to get our understanding of the virus up to a basic level, and know how to keep ourselves and our loved ones safe."
Professor Jane Anderson, chair of the British HIV Association, added: "Many people are simply not being signposted to take an HIV test.
"With a general lack of routine HIV testing being commissioned for general medical admissions and in the general practice setting this is not altogether surprising.
"We need a greater emphasis on HIV testing and on all aspects of prevention.
"With successful treatment, a person with HIV in the UK can expect a near-normal lifespan.
"But for that to happen, early diagnosis is vital.
"Too many people only get tested when their HIV infection is already at an advanced stage, compromising both their own health and that of their partners."
HIV attacks the immune system and weakens the ability to fight infections and disease.
AIDS is the final stage of HIV infection, when the body can no longer fight life-threatening infections.
HIV is found in the body fluids of an infected person, and about 95% of people are infected through sexual contact.
Deborah Jack, chief executive of the National AIDS Trust, said: "What is striking about the HPA's data is how it really shows both our successes and our shortcomings in tackling HIV in the UK.
"On the one hand, we can hail treatment as a real success story. Treatment is effective, people diagnosed with HIV can access it easily, and it is working in keeping the virus under control.
"However, when it comes to increasing the uptake of testing - the gateway to treatment - our services are patchy, inconsistent and ultimately we are still failing to make any significant headway in tackling the high rates of undiagnosed HIV."
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