... a condom, which also prevents STDs (which can't pass through the little holes, as some would have you believe)
But can pass through the surprising number of larger holes that are 'fabric' failure. Heard a doctor on this a few months ago and he reckoned that somewhere in the region of 10-15% of condoms fail on this score. He referenced a stress test that is done by a group like 'Which?', but which wasn't 'Which?'