What would you call this illness?

A stomach virus?

If it also involves some heavy vomiting, it sounds close to old sailor’s disease (don’t know the actual name of it, just that the symptoms you described are somewhat similar to what I have had twice but I was also vomiting a lot as well for the first day, day and a half).

Sounds like norovirus.

In the First World War I read soldiers were often diagnosed PUO or pyrexia of unknown origin. It usually cleared up in a few days.

Influenza. It’s going around big time right now.

If “la febbre” is what is known in Spain as “la fiebre”, then it is just a generic name for any kind of strong cold or flu.

Well, flu knocks you on your ass for a 4 or 5 days and high fever. But, adults rarely have a lot of vomiting with the flu.

Once I had a terrible flu sick for about a week fever vomiting. I went to the doctor and they told me I had the flu.