why do some viruses cause only minor problems, while others can cause
widespread illness and death?

how wide spread it is depends on how quickly the virus can reproduce,
and by what medium it is spread by(air, water, physical contact).
the effect it has on an individual(death or not) depends on the
strenght of that person's immune system at the time of infection and
what bodily function the virus effect.

This knowledge was taken from what i can remember of my lecture notes,
which i'm currently revising at the moment(i'm on a break!!) for my
summer exams.