Flu 2025 Symptoms. The two main types of influenza viruses—types a and b—are the viruses that typically spread in people (human influenza viruses) and cause annual flu outbreaks. Influenza, or the flu, is both a seasonal and a pandemic virus.
In contrast, other viral forms, such as rhinovirus, only cause a common cold or milder symptoms. And, of course, how can you tell.