A south London teenager who died on Wednesday was the fourth person in Britain to succumb after contracting swine flu.

The 19-year-old, who had serious, unspecified medical problems, tested positive for the influenza A (H1N1) virus following his death in hospital.

He was the first person in the capital to die with swine flu, following three other deaths – two in Scotland and one in Birmingham.

London, along with the West Midlands and the Glasgow area, is a hot spot for transmission of the H1N1 virus. Everyone has a responsibility to follow government advice on minimising the risks of passing on the virus to vulnerable people, said Simon Tanner, regional director of public health in London.

"There are people out there in the community, and we've talked about one today – precious individuals that have got really difficult medical conditions – who are particularly vulnerable when the population has a virus like this in it," Mr Tanner said yesterday.

"We've all got a responsibility for their sake," he said. "We would make the judgment that a major conurbation, with a lot of people coming into it every day, with lots of people working in close proximity, is likely to be the sort of area where you would see the spread of any infectious disease."

Yesterday's news of the teenager's death followed a warning by Sir Liam Donaldson, the Government's chief medical officer, for the public not to buy counterfeit anti-swine flu drugs over the internet.

Sir Liam said there was a government stockpile of Tamiflu, the main antiviral drug, ready to be distributed through the NHS. "There's generally a growth in people ordering drugs from the internet worldwide and there's a lot of concern among health authorities that people might buy counterfeit drugs," Sir Liam said.

"We have got a massive stockpile [of Tamiflu] in this country, and everybody can have access to it through the National Health Service," he added.

Health officials in Hong Kong said yesterday that they had detected a case of Tamiflu-resistant virus in a 16-year-old girl. The girl recovered without medication and the resistance was detected in routine tests.

On Thursday, the British Government warned that, at the current rate of spread of the H1N1 virus, the number of people who are contracting the virus could reach as many as 100,000 people a day by the end of August, leading to a full-scale flu epidemic this winter in Britain.

Sir Liam said the Government's policy of trying to contain the virus has changed to one of treating only those people who become infected.

"We have been dealing with it very aggressively so far – we have investigated every case, we've treated their contacts, we've closed schools and all of that has helped slow the spread," he said.

"We are surprised we have been able to run it as long as we have like this. Flu viruses spread extremely quickly so this is very much going as expected," he added.