The first case of Ebola was recorded in the US last week.
Thomas Duncan arrived in Dallas, Texas, from Liberia, west Africa.
He went to hospital when he began to feel ill, but was turned away with antibiotics.
This put others at risk of infection and Duncan is now gravely ill in hospital.
The disease is unlikely to spread in the US.
It is in desperately poor African countries that more than 3,000 are now dead. The number of cases doubles every few weeks.