People are more likely to conceal their sicknesses when their risk of infecting others is higher, a University of Michigan study found.

Out of 4,110 participants, 75% have hidden having an infectious illness or would do so in the future.

Those surveyed admitted to boarding planes, going on dates, and socializing while sick. 61% of those working in healthcare said they had hidden an infectious illness.

“Actively sick people reported high levels of concealment regardless of how harmful their illness was to others,” lead author Wilson N. Merrell said. People may hide sickness to avoid being perceived as weak or lazy, or to avoid isolation.