It's mentioned in many places, such as Mäkelä et al. (1998) that Influenza viruses are among the many viruses that cause the Common Cold (other viruses include Adenoviruses, Rhinoviruses, Coronaviruses, RSV, etc.).
Now, Influenza viruses are already well-known for causing their own characteristic clinical disease, Influenza ("the flu").
What determines whether an individual Influenza virus, upon introduction into the human body, will cause a clinical case of influenza or the common cold?
- Are there separate "flu-causing" and "cold-causing" strains of Influenza virus, and which one someone gets will determine which clinical disease they develop? In other words, if you get exposed to a "cold-causing" variant, then the chances that you will develop "the flu" from it are essentially nil. Instead, you will develop at cold or possibly nothing at all.
- Is it dependent on entry-point into the body (e.g. nose versus throat)?
- Is Influenza the disease simply a shorthand for a disease caused by an Influenza virus that happens to clinically present as worse than a cold? In other words, this would be equivalent to claiming that mild, near-asymptomatic COVID-19 and the kind of COVID-19 that puts someone on a ventilator are completely separate diseases that can both be caused by SARS-CoV-2 virus.
- Is it based on some other factor(s)?