One particularly bad virus is cassava brown streak disease.
It began infecting cassava fields in East Africa 10 years ago.
Now it has spread as far west as the Democratic Republic of Congo.
Brown streak disease is spread in two ways: by white flies and by infected stem cuttings.
Farmers use these cuttings instead of seeds to plant their fields.
Claude Fauquet is a plant virus expert.
He heads the Global Cassava Partnership for the 21st Century.
He says it is hard to see signs of the disease in the plant itself.
Instead, it becomes visible in the roots when the plant is harvested.
He says scientists and organizations have to find a way to offer farmers virus-free plant cuttings.
Claude Fauquet says scientists are experimenting with a virus-resistant cassava plant in Tanzania.
Experts warn that brown streak disease could reduce cassava production in Africa by 50 percent.