This worm, called schistosoma, is actually very tiny. It’s been greatly enlarged in this photo. It’s a human parasite that spreads in contaminated water. It can penetrate pores in the skin of people who wade or swim in the water. Worldwide, infections with the schistosoma parasite are common. This poses a serious problem, because the parasite makes people very sick.
Like this parasitic worm, many other organisms can make us sick if they manage to enter our body. Fortunately for us, the body can defend itself from most such invaders. In this chapter, you’ll learn how.