[Gibbon1]

I keep seeing reports that after an outbreak has played out about 25% of infected never show symptoms and 60% have something between a mild cold and a bad case of influenza. The remaining 15% get really sick and ~1% die.

If you just look at the stats initially you run into the problem that a lot of infected are preasymptomatic are in the early course of infection. So retrospective stats are very important.