Symptoms can be missed
HIV is being spread because doctors overlook symptoms which could reveal the infection, a charity claims.
The National Aids Trust said as many as half of all early-stage infections, often marked by severe flu-like symptoms, are being missed.
Spotting them and carrying out an HIV test would prevent further infections, it said.
A GP specialising in sexual health said doctors should always be open-minded to the possibility their patients had HIV.
There are approximately 7,000 new HIV infections in the UK every year, and as many as 50% are estimated to be passed on by people who are in the early stages of their own infection.
In the first few weeks after infection, there are massive levels of the virus in the blood, and in most cases, this causes symptoms such as sore throats, fever and rashes. A person with HIV is at their most infectious at this point.
However, after six weeks, these symptoms generally recede and the infected person will feel back to normal, even though they still have HIV.
However, the National Aids Trust (NAT) said that people visiting a doctor, either their GP or in A&E, complaining of these symptoms were often told it was a trivial viral infection, and to return if it did not improve.
A study in Brighton found that 48% of HIV patients who had sought medical advice with their early symptoms had not been diagnosed.
Deborah Jack, the chief executive of NAT, said: "It is very worrying that GPs and other healthcare professionals are often missing the signs and symptoms of HIV infection.
"This can mean they become seriously ill in the longer term and respond less well to treatment.
"It also means they are likely to be putting partners at risk of infection as they may live undiagnosed for a number of years."
Dr Martin Fisher, a consultant in HIV medicine, said that this brief period was a "golden opportunity" to spot new cases.
He said: "HIV testing needs to be more widespread and routine. It's reasonable to expect doctors to be able to make this diagnosis."
Dr Christian Jessen, a GP specialising in sexual health medicine, said that doctors were still guilty of being influenced by the stereotypical notion of the "gay man with HIV".
"I have seen so many cases come to me which have been missed, and people with HIV are not just gay men, they are heterosexual men and women as well.
"Doctors need to always be alive to the possibility that the person in front of them may have HIV."
Lisa Power, of the HIV charity Terrence Higgins Trust, said it was important that those of particular risk of HIV, and those who give them healthcare, needed to know the signs of early infection.
"Sore throat, fever and a rash? Go and get it checked out, and make sure the various checks includes an HIV test."