Probability 101 and Face Masks

Proposition: face masks are effective when you're sick, but aren't when you're healthy.

Is this not a basic misunderstanding of conditional probability? The prior of coming into contact with a sick individual is sufficiently low such that yes, whether you wear the face mask or not, you're unlikely to get sick walking in public. Conditioning on your being sick however, you bet your a we're gonna insist you wear that mask!