The unexposed aren't at zero risk, but are often quite close. For example, cervical cancer without HPV infection is extremely rare, but non-zero. Or someone at

The unexposed aren't at zero risk, but are often quite close. For example, cervical cancer without HPV infection is extremely rare, but non-zero. Or someone at the party who didn't eat the potato salad could still come down sick by another pathway. On the other hand, for things like environmental exposures, you're often looking at very small increases in risk over baseline. For example, your increased risk of asthma with a higher increase in particulate matter in the air.