An Illinois businessman who contracted the mysterious Middle East virus know as MERS is believed to be the first case of person-to-person transmission inside the United States.
Federal health officials say the businessman was likely exposed to the virus when he shook hands during two business meetings with a man authorities are calling "Patient No. 1."
The man from Illinois is a health professional who worked with MERS patients in Saudi Arabia and brought the disease stateside last month.
He flew from Jeddah to London to Boston to Atlanta, then finally to Orlando, coming in contact with 500 travelers on his U.S. flights alone.
The situation has sparked new concerns about the potentially deadly respiratory virus.
"This virus is more easily transmittable than we thought initially," Dr. LaMar Hasbrouck, with the Illinois Department of Public Health, said.
The Illinois man initially tested negative. But a follow-up test now shows his body fought off the potentially fatal disease.
"Every single case is different," Hasbrouck explained. "It is a spectrum of possibilities, and we're trying to learn as much as we can."
The Illinois man is wearing a mask at home and avoiding contact with his family.
Now investigators are contacting his family, friends, and those who came in close contact with him while he was infected, including people in 39 states who shared flights with him.
Tests should soon determine if he transmitted the virus to anyone else in the United States.
With travel season picking up, there is growing concern that the virus could soon spread from the Saudi Arabian peninsula.
"Travel to the peninsula is about to kick off. We have important pilgrimage holidays and Ramadan coming up at the end of June," Dr. Marty Ceron, with the Centers for Disease Control and Prevention, said.
The disease has killed more than 160 people in the Middle East.