For a vaccine to be effective, it must do two things. First, it must trigger an immune response. Second, the vaccine must train the body to remember the response so it can fight that same disease in the future.

Schistosomiasis is caused by a worm that contaminates fresh water. The worm’s larvae penetrate the skin and develop into adults. Those adult worms produce eggs that cause the disease. Schistosomiasis can be found in nearly 80 countries and is common in sub-Saharan Africa. The World Health Organization estimates 250 million people are carrying the disease. An additional 800 million people are at risk of getting the infection, making schistosomiasis second only to malaria among the world’s deadliest tropical parasitic diseases.