According to the Trade-Off Hypothesis, Why Are Diseases Like Mumps

According to the trade-off hypothesis, why are diseases like mumps, measles, or chickenpox rather mild?

A) The viruses reproduce very quickly and die before symptoms get too bad.
B) Mostly children get these diseases.
C) They're highly contagious already.
D) They require direct human-to-human contact to spread so the human hosts can't be too sick to get around.
E) They require direct contact with infected insects so the human hosts can't be too sick to get around.