Two American aid workers have contracted the deadly ebola virus in Liberia as the outbreak continues to worsen in West Africa. Over 670 people have died since March in Guinea, Sierra Leone, and Liberia. NPR reports that the virus has probably now spread to Nigeria.

Patrick Sawyer, a 40-year-old Liberian finance ministry employee, may have infected his fellow air passengers when he flew from Liberia to Nigeria on Friday while vomiting from the virus. CBC News reports:

Witnesses say Sawyer ... was vomiting and had diarrhea aboard at least one of his flights with some 50 other passengers aboard. Ebola can be contracted from traces of feces or vomit, experts say.

Sawyer stopped in Ghana and changed planes in Togo on his way to Nigeria. He was immediately quarantined when he arrived in Lagos.

Both Americans—Dr. Kent Brantly from Texas and Nancy Writebol, a missionary from North Carolina—are still alive in Liberia. Brantly is in "stable condition," but Writebol seems to be doing worse. Her pastor told The Charlotte Observer, "She's not doing well. It's grim news."

There's only a remote chance that the virus could spread to the U.S.

[Image via AP]