Question

(a) how do houseflies carry disease-causing microbes (or pathogens)

Answer 1

Usually Houseflies sit on faeces, corpses or decaying food matter which houses various pathogenic micro-organisms. When they sit on those materials, the pathogens stick to the legs/appendages and bristles
Then they are transmitted to the host.

But sometimes flies use saliva to soften the hard foods as well.. So sometimes microbes residing in the intestinal tract are promtoted as well.

Answer 2

As much as I know they carry microbes and pathogens in their feet i.e. they carry microbes bcoz they fly dirty places too and so we are prohibited not to eat open foods