That gives you more than a 50% chance of getting sick, calculate environmental health experts Mark Nicas, PhD, of the University of California, Berkeley, and Rachael M. Jones, PhD, of the University of Illinois, Chicago.
But if that doesn't happen, what's the next most risky thing?
Touching something contaminated with flu virus and then touching your nose, mouth, or eyes with your unwashed hand gives you a 31% chance at getting sick, Nicas and Jones calculate.
Breathing in tiny particles left hanging in the air from a flu-infected person's cough or sneeze gives you a 17% chance of infection. Breathing in larger particles -- which hang in the air for a shorter time -- gives you only a 0.5% chance of getting sick.
The calculations are based on a scenario in which a family member is taking care of someone sick in bed with a type A flu bug. H1N1 swine flu is one such virus.
Can you count on these numbers to keep you safe? No, Nicas and Jones admit. The calculations are based on many factors and situations -- such as the amount of virus in an infected person's body or the humidity of a room -- that change from day to day and from person to person.
"As a result, we conclude that nonpharmaceutical interventions [to prevent infection with] a pandemic virus must account for all routes of exposure," Nicas and Jones note in their report, published in the September issue of Risk Analysis.
There are four ways to catch the flu, including H1N1 swine flu:
Touching a surface contaminated with virus and then touching your face.
Breathing in tiny droplets containing flu virus dispersed in the air.
Breathing in medium-size droplets containing flu virus, which do not travel as far or hang in the air as long as tiny droplets.
Having large droplets deposited directly onto your facial membranes.
"More reliable information concerning these areas would lead to a less uncertain apportionment of influenza infection risk among the four exposure pathways," Nicas and Jones conclude.