> 4. (This is the big one) [...] People with the flu tend to self-isolate showing symptoms.

Even if optimistically 80% of people do that (in western europe I'd guess it's more like 45%+/-20, might be better elsewhere), if the spreading ability is high (points 1 and 2), you get a bus full of people infected by the two out of ten individuals that decide calling in sick isn't worth it

Technically you can only call in sick when you are literally not able to do your job, and that's not the case if you're just coughing and feel cold or so. Even if your employer might prefer that you don't take the whole team down with you, people's judgement seems to very much be on the "it'll be fine" side. Idk that this is 'the big one' outside of a 2-out-of-100 years pandemic situation where people are exceedingly careful and paranoid