He just had the common cold; a flu is serious, and you don't get the cold or flu because you have been in the rain that is an old wife's tale.
Influenza A is a type of virus that causes influenza (the flu), a highly contagious respiratory illness. If you get it, you will need to rest at home and avoid infecting others.
I have to add where I live fireworks are prohibited for private persons to obtain, and yes, we have fireworks certain days of the year, but they done by pyrotechnician, and the fireworks are stored well away from housings.