The overall risk of contracting a disease from an ill person on board an airplane is similar to that in other confined areas with high occupant density, such as a bus, a subway, or movie theatre for a similar time of exposure. anywhere where a person is in close contact with others.
That said, the risk on airplanes is probably lower than in many confined spaces because modern airplanes have cabin airfiltration systems equipped with HEPA filters.
HEPA or high efficiency particulate air filters have similar performance to those used to keep the air clean in hospital operating rooms and industrial clean rooms. These filters are very effective at trapping microscopic particles as small as bacteria and viruses (taken from IATA Corporate Communications)
