MILWAUKEE _ Epidemics start in mundane ways.
A child might play with a pet. An air-conditioning unit might break down. A pool of water might collect in an empty flowerpot.
Any one of those actions is all it takes for a virus to find a host and infect its first case.
From there, the virus can spread exponentially _ across cities, regions, countries. Millions of people might be exposed from that single event, and thousands might die.
This year, the Western Hemisphere is reeling from an outbreak of the mosquito-borne Zika virus, which has been linked to unusually small heads and brain damage in newborns.
It prompted the World Health Organization to advise Brazilians to consider delaying pregnancy. And it is overshadowing the Summer Olympics set to start in Rio de Janeiro next month.
On July 8, the first death in the continental United States from the virus was confirmed.
But Zika is just the latest of many.
Since 1980, the global number of viral outbreaks per year has more than tripled. In the last three years alone, health officials have confirmed outbreaks of dengue, chikungunya, measles, enterovirus, Middle East respiratory syndrome (MERS) coronavirus, and Ebola _ and those are just the ones that have reached the United States.
Then there are the viruses that haven't emerged yet. The World Health Organization has listed Nipah, a virus similar to the fictional subject of the movie "Contagion"; the MERS and SARS coronaviruses; and several hemorrhagic fevers, including Ebola, as the viruses most likely to cause a major epidemic in the near future.
But predicting potential outbreaks is challenging because so many factors are involved.
"How do you figure out what's coming next?" said David O'Connor, professor at UW-Madison and chair of the Global Infectious Disease Division at the Wisconsin National Primate Research Center. "That's the million-dollar question."
What we do know is that epidemics can affect virtually anyone.
