It is quite simple math that is at work. The first cases were seen in early March. If you assume that each victim spreads it to just two more people in about a week, then cases would triple each week (including the original victim each week) . That would work out roughly to the 2,500 now suspected cases in Mexico. Tripling each week would mean about a million and a half cases in three months, rising to 6 billion or so in another eight weeks, i.e. the world's population, roughly. This is less time that it will take to manufacture a vaccine. Even with a widely used vaccine, ordinary flu causes from a quarter to a half million deaths world wide each year. The fatalities from this flu would likely be higher just because of the lack of vaccine, even if it turns out to be no more deadly than normal flus. But the apparently excess deaths in Mexico indicate it might be even more dangerous than that.