People over 50 are as much at risk from unsafe sex as the young, the Health Protection Agency warns today, as it reveals evidence that the numbers becoming infected with HIV in that age group have more than doubled in seven years.

In 2000 there were 299 new HIV infections among the over-50s, according to the HPA, which presents its findings at the International Aids conference in Vienna today. But in 2007 there were 710 new cases.

Some of those diagnosed after the age of 50 will have been infected when they were younger and diagnosed late. After some early symptoms, people who are fit and healthy often continue to feel well and may not suspect they have HIV for as long as 10 years. Unfortunately, late diagnosis reduces their chances of survival.

But half of the over-50s diagnosed had recently been infected, suggesting they had been taking chances.

"This highlights the importance of HIV testing, whatever your age," said Ruth Smith, a senior HIV scientist at the HPA's Centre for Infections. "We must continually reinforce the safe sex message – using a condom with all new or casual partners is the surest way to ensure people do not become infected with a serious sexually transmitted infection such as HIV."

Her co-author, Dr Valerie Delpech, head of HIV surveillance at the HPA, said people in the older age group needed to be aware that they were just as much at risk as young people if they had unsafe sex.

"Although adults aged 50 and over account for just 8% of all new HIV diagnoses, the fact that cases have more than doubled in recent years serves as a timely reminder that anybody is at risk of HIV infection if they do not use protection and practise safe sex," she said.