incubation and contagious periods for common diseases
The contagious period is the amount of time during which a sick child can give the disease to others. For major illnesses (such as hepatitis), a child will need to stay at home or in the hospital until all chance of spread has passed. For minor illnesses (like the common cold) the guidelines are less strict.
The incubation period is the time between being exposed to a disease and when the symptoms start. If your child was around someone who is sick and the incubation time has gone by, then your child was probably not infected and won't get sick.