Ebola, the deadly virus that has killed more than 800 people and infected thousands in West Africa, has arrived in the United States.

Missionary Nancy Writebol, the second U.S. aid worker who contracted the Ebola virus, is anticipated to arrive in serious condition in Atlanta on Tuesday, August 5. The first was U.S. doctor Kent Brantly, who traveled to West Africa on an emergency response to help with the outbreak. Brantly arrived to the U.S. on August 2 and is already being treated by infectious disease specialists at Emory University Hospital, where Writebol will also be treated.

Writebol and Brantly are believed to be the first ever Ebola patients to be treated in the United States.

The outbreak of the virus started in February and has been spreading since. Ebola is an unpreventable and contagious viral disease that spreads through bodily fluids. Currently, there is no cure. However, ZMapp, an experimental drug, was used on both Brantly and Writebol, which improved their conditions. Zmapp was created by Mapp Biopharmaceutical Inc., a biotech firm in San Diego, and has never been tested on humans, only monkeys, until now.