New figures show almost half of people who have HIV in England, live in the capital.
That same study shows that the number of gay and bisexual men being diagnosed with HIV reached an all-time high last year.
Nick Thatcher reports.
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Paramedics were on standby outside the police cordon as armed officers stood by the property.
A team of researchers has taken samples from the ten busiest London Underground lines to work out which are the dirtiest.
The action comes on top of an emergency timetable which has seen hundreds of trains a day being axed.