The flu is contagious and can easily spread in crowded places.

Able to be spread from one person to another, typically referring to diseases.
Some infections are highly contagious.
It's important to stay home if you're contagious.
Doctors advise caution when dealing with contagious illnesses.
contagious disease
contagious infection
contagious period
infectious
transmissible
communicable
The flu is contagious, so wash your hands often.

Able to be spread from one person to another, usually referring to diseases.
Chickenpox is highly contagious, so stay away from infected people.
The doctor said the rash is contagious and advised us to be careful.
During the outbreak, many schools closed to prevent the spread of contagious illnesses.
contagious disease
contagious infection
contagious illness
infectious
transmissible
communicable