That would probably be the case of only one child became infected, and that child in infected no one else. However, if there are a sufficiently high number of

That would probably be the case of only one child became infected, and that child in infected no one else. However, if there are a sufficiently high number of susceptible people, an epidemic can occur (i.e. an event where an exponentially growing number of people become infected.) That's really what we worry about.