Nancy Writebol, an aid worker in Liberia, was helping treat Ebola patients until she tested positive for the virus herself - the second American so far.
This comes just days after another American, Dr. Kent Brantly also became infected while treating Ebola patients.
The current outbreak is the deadliest ever, so far claiming hundreds of lives.
And many more people could be infected and not even know it yet. One man boarded a flight last week not knowing he had Ebola. But days later, he died. His flight included stops in three countries.
CNN's Erin Burnett speaks with Ken Isaacs, Vice President of Samaritan's Purse, an organization the two Americans are affiliated with - about their current condition.