Some of these small parasites have been mutating and are now resistant to many over-the-counter medications.
In the next two weeks, more than 30 marathons are scheduled in the United States, in cities big and small.
Among them is the Oklahoma City Memorial Marathon, scheduled for Sunday, April 28.
Since 2001, the Oklahoma City National Memorial and Museum has held the marathon to commemorate the bombings in that city, which killed 168 people and injured more than 800 others.
It will be eighteen years since the bombing on Friday, April 19, 1995.
But given the events at the Boston Marathon, Oklahoma City marathon organizers are cautiously evaluating security plans and making their decisions one day at a time.
More than 20,000 people are expected to run the race there.