In general the more severe a virus the less likely it is to spread into a pandemic. This is for a couple reasons: 1) really sick people are less likely to go spread it, 2) really sick people are easier to identify and isolate and 3) the time between virus shedding and death is very small.

There are, of course, some exceptions (long incubation periods where you can shed virus but are totally asymptomatic) but those are rare and usually self limiting in some other factor. Smallpox was really the worst possible case and we have thankfully eliminated it.

Big deadly pandemics (black plague level) are usually bacteria.