I'd think twice before I'd knowingly share my property with rats. The
rise of hantavirus is certainly reason for one to pause (paws?), and just
because it has not yet turned up in one's local area does not justify
smugness. While not known to be ratborne, we're still on the learning
curve with this one.

Here in western Washington, we have only the black (roof) and brown
(Norway) rat species, but both share a tendency to invade our dwellings
once they figure out that it's warm in there and there is also food
inside. The closer one lets them exist, e.g., in one's yard or compost
pile, the more likely a domestic incursion. Also, the bubonic plague
organism exists in wild rodents in eastern Washington and much of the
West, and while rats are not the only animal with fleas, letting them
exist increases the probability of one's yard having a higher flea density
than otherwise, with the possibility of disease transmission. I really
dislike fleas and their bites.

Letting rats harbor on one's property is defined as a nuisance under many
municipal ordinances, and because rats do not respect property boundaries,
you may cause neighbors problems, especially when rat populations peak
seasonally and they disperse to find new homes as they do annually. The
first rat survey I ever did as an incipient environmentalist began at a
feeder in Minneapolis, and by the time the campus sanitarian and I were
finished with the block, I couldn't believe how many burrows we had found
and how many rats they must have represented. But we had to *look* for
them because they were pretty well concealed.

When rats visit my feeders, I trap them with snap traps concealed such
that birds and non-target wildlife are unlikely to find them, and I use
baits. Poison baits can be used similarly, e.g., inserted into their
burrows where there is little risk of accidental ingestion; the rats thus
bury themselves when they die.

Indeed, the rat, not the dog, is man's oldest if not best friend, and it
makes no sense to me to tolerate them in any numbers. There comes a time
when we must define sharply our best interests as humans, and history is
replete with examples of what happened in societies which failed to do so.