De Blasio’s comment reminds me of what an AIDS educator once told me was his advice to gay men: “When asked if you are HIV+, for many of us there are only two possible answers: ‘Yes’ and ‘I don’t know.'” From the SF AIDS Foundation:
There’s a period of time after a person is infected during which they won’t test positive. This is called the “HIV window period.” The window period can be from 9 days to 3-6 months, depending on the person’s body and on the HIV-test that’s used. During that time, you can test HIV negative even though you’re HIV infected. You can still catch HIV from someone who is in the window period. In fact, there is evidence that a person in the window period is more likely to pass the virus on.
It should go without saying that the HIV window should always be kept in mind when you see personal ads trumpeting recent negative test results.