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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)?

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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?

Pretty much. You have to bear in mind that until the last 25 years or so there were no practical laboratory tests to distinguish among the various viruses causing influenza or the common cold. Viral cultures or PCR might have been possible in a research setting, but were too expensive and required too much specialized training to be practical in the clinic.

Colds and flu were mostly distinguished by the severity and course of the disease. Influenza tends to come on suddenly and is often accompanied by fever. Colds generally resolve in a couple of weeks without complications. Influenza can quickly result in life threatening complications. If you just have a runny nose, cough, and sore throat, you might have a cold or you might have a mild case of the flu. If you also have a fever, prostration, and develop pneumonia, then you most likely have the flu.

The severity of the flu depends on the exact strain and your immune system's previous exposure to it.

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