I would add to what John said. The picture emerges over time. The H1N1 virus is a good example. When it first emerged in Mexico, it appeared to be a pretty severe problem because the most severely ill people are the ones who come for medical attention first. But as things evolved we learned that it could be severe, but most cases were pretty mild and got better fairly quickly. It's a constantly changing picture, and you don't know when you're starting. You can make some guesses based on past experience, but you don't know until you've actually seen what's happening.
On October 2nd, 2012. See this statement in context.