“You could see little parts of the human cell being broken off,” Ralston said of her very first encounter with the parasite.

The U.S. Department of Agriculture said May 11 that it is suspending imports of livestock through Mexico because of New World Screwworm threats.

The single-celled parasite Entamoeba histolytica infects 50 million people each year, killing nearly 70,000. Usually, this ...