SFO — Passengers on a flight from Philadelphia to San Francisco on Saturday were exposed to tuberculosis when a sick man on a no-fly list was allowed aboard, but the risk of the infection spreading is low, officials said Tuesday.

The unidentified man had been placed on the “Do Not Board” list one day earlier because of his TB, according to Christine Pearson, a spokeswoman for the federal Centers for Disease Control and Prevention.

It wasn’t until after the US Airways flight took off that authorities discovered the sick man was aboard, Pearson said. San Mateo County health officials met him at SFO, isolated him and took him to a hospital.

US Airways is reportedly contacting passengers on that flight. According to a report on MSNBC, the man boarded US Airways flight 401 bound for San Francisco around 6 p.m. Saturday.

The CDC collects information from state and county health departments and adds to the “Do Not Board” list people afflicted with dangerous, contagious diseases. The goal is to prevent the spread of infection in the controlled air environment of an airplane. That information is then relayed to the federal Transportation Security Administration, which notifies the airlines.

TSA spokeswoman Sterling Payne said the agency had done its job and the airline had access to the information about the man with tuberculosis. US Airways did not immediately return a phone call asking how the man was able to board the plane.

Tuberculosis is a potentially deadly infectious disease that typically attacks the lungs. Symptoms include a persistent cough with blood, chest pain, night sweats and fever.

What makes the disease dangerous is the fact that most symptoms don’t appear until the disease is too far along for treatment to be effective. Treatment could last for six months or longer.

This oversight is just the latest mishap to plague Philadelphia International Airport and the TSA.

Last week, a plane was returned to the gate after the luggage of a suspicious man — who was arrested before boarding — was left onboard. Passengers forced officials to turn the plane around.