Given the close quarters over extended time periods, airplanes may seem the perfect environments for the spread of communicable disease. However, there is actually surprisingly little data on the question, according to doctor Andrew Pavia, writing in the Journal of Infectious Diseases.

While it does happen, airplane-linked contagion can be difficult to trace, especially for respiratory conditions, which are typically spread by small airborne particles. Pavia notes that efforts to trace transmission of the SARS virus between airplane passengers were inconclusive.

The type of plane matters. In three studies of flu outbreaks traced to aircraft, two occurred on older planes with poorly functioning air-circulation systems. Modern aircraft continuously circulate the cabin air through filters, in theory preventing most small particle dispersal. But according to Pavia, there are few regulations regarding filter maintenance.

Where epidemiological information is known, the risk of transmission varies considerably depending on your seat location. During a notorious 2007 incident aboard a transpacific flight, one passenger boarded the plane sick with a viral gastrointestinal (GI) illness and infected 52 others during the flight. Most infections occurred among those who were sitting near to the primary patient or who shared a restroom with him. The cramped, awkward layout of aircraft restrooms is not conducive to thorough hand washing, and the restroom handle was a likely culprit in several cases. GI illnesses can be spread by vomit, where the heavier particles settle onto nearby surfaces. On the bright side, the air filtration seemed to work, as few people seated far from the patient fell ill.

The bottom line is that the risk of airborne viral infections exists but is limited. However, GI infections pose a documented risk to air travelers.