In old houses if you had a hot water storage tank in the attic it was usually just covered with styrofoam or a metal or wooden slab, it wasn't uncommon for an occasional insect or yes, a small rodent to fall in and die in there. It's one of the reasons why most houses in the UK had separate cold and hot water taps - cold water was considered safe to drink, hot not so much.
Nowadays if you have a hot water cylinder it's impossible for anything to get in there(although there is still a chance of bacteria growing inside the tank, but any modern cylinder will periodically heat itself to very high temperature to kill any pathogens)