At the same time, the virus has begun to invade more species, including mammals - infected cats, pigs and dogs have been reported.
Most infected cats have been healthy before a very sudden onset of severe disease.
All infected cats require supportive care, including careful fluids, nutritional support, treatment for complications, and often blood transfusion.
A single infected cat can shed 100 million oocysts in its droppings.
With proper care, infected cats can live long and healthy lives.
Most infected cats will shed oocysts only once in their lifetimes, for a period of about one to two weeks.
Currently, there are no drugs to help infected cats remove all of the worms.