Hand-foot-and-mouth disease is most contagious during the first week after the onset of the illness, according to the Centers for Disease Control and Prevention. The risk of transmission is still likely for days or weeks after symptoms subside.
Some adults may carry the viruses that cause hand-foot-and-mouth disease without showing any symptoms, the Centers for Disease Control and Prevention details. The illness is highly contagious when in close contact with bodily fluids from an infected person. This includes breathing in the virus from the air, coming in contact with saliva or feces, or touching contaminated surfaces such as doorknobs and toys.