Single-Celled Organisms

Single-celled parasites undergo life cycles as complex as those of much larger worms. Like these, the single-celled organisms multiply in the intestine, blood, or tissue and can grow there in huge numbers. They often cause chronic and even fatal infections, against which drugs can help, yet hundreds of millions of people are newly infected each year. The pathogens presented are transmitted through contaminated food, by insects, or sexually.