Behavioral contagiousness
That the smile is contagious (in the sense that our laughter is able to generate good humor in the people who look at us, and vice versa) is certainly not a novelty.
Recently, however, scholars have confirmed that infectiousness is such also for other bodily noises and attitudes, including yawning and retching.
Seeing a person vomiting, therefore, tends to trigger vomiting even in the beholder.
Microbiological contagiousness - Infections transmitted through vomiting
Beyond the behavioral aspects, vomiting can be contagious even in the microbiological sense.
A typical example of a disease transmissible through vomiting is given by viral gastroenteritis (such as those sustained by norovirus), due to the presence of viruses in the regurgitated material.
If vomiting contains visible traces of blood (hemorrhagic vomiting or hematemesis), and comes into contact with a wound, eyes or mucous membrane, it can also be a vehicle for various other diseases, such as ebola or AIDS.
