Answer:
Due to strong immune system and dormant state.
Explanation:
If a person who looks well could still be a source of infecting  microorganisms because the microorganisms present in its body not affected him due to powerful immune system but it can cause infection in the body of those person who have weak immune system. The other reason is that microorganisms are present in dormant state due to unavailability of good growing condition but when they got good conditions they became active and causes infection.
