The main way Zika is spread is by mosquitoes.
But if Zika can also spread through sex, then it poses a risk to every country not just those with the Aedes mosquito.
So far, authorities have said sexual transmission is rare, but last year they would have said any case of Zika was rare, too.
This explosive outbreak has caught the world by surprise and many key questions remain unanswered.
Exactly how common or rare is sexual transmission? Can it be spread by the 80% of people who show no symptoms? How long does the virus persist in semen? When is it safe to have sex again?
What should men do after visiting affected countries? Can women also spread the virus through sex?
However, this is not a new HIV/Aids moment. HIV infection is incurable and dramatically shortens lives without daily medication.
Zika infections are short, mild and pose a significant threat only in pregnancy.
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