Some viral infections, such as colds and the flu, are very contagious, according to Healthline. This means that the infections can spread through the respiratory system or through physical contact with another individual.
Other types of viral infections, such as hepatitis C, are only spread through direct contact with another individual's blood, the American Liver Foundation states. Similarly, HIV is most commonly spread through sexual contact or using infected needles to inject drugs, according to the CDC.
It is especially important for an individual who has the flu to stay home and avoid spreading the virus, Healthline states. This is because the flu can cause serious complications, such as pneumonia, in certain individuals.