What Agents Cause Colds and Flu?

Influenza is caused by a virus (specifically, it’s called an orthomyxovirus). There are three types of influenza viruses: A, B, and C. Only types A and B infect humans and only type A has been associated with serious epidemics, or outbreaks, of disease.

Influenza viruses are unique: they constantly evolve and many different strains of flu can arise from one virus. For this reason, the flu virus that affects a community one year is usually different than the one that affected that community the previous year. Since these flu strains are different, the protection we may get from our immune systems one year (for example, after getting a flu shot or having a case of the flu) usually does not protect us from the flu the next year. This is why people need to get a new flu shot each year.