The Mayor of London Boris Johnson said: "London is home to almost half of all people living with HIV in the UK, but a quarter of them are unaware they carry the virus.
"It is vital that people who might be at risk get tested, not only to reduce the risk of transmission to others, but to ensure they get the life-saving treatments that are available."
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A volunteer on her campaign team said to a voter that rival Sadiq Khan was a "liability" because he is Muslim.
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The demonstration is due to take place next Saturday in central London ahead of the "emergency" meeting in Brussels to discuss the crisis.