Why wouldn't some kind of a plague hit 50% of all races, killing 25% of population instantly and killing more per turn, so other races would have a choice:
a) terminate all trading with plagued races and don't get plague (possibly have a risk because of the tourism and smugglers, maybe 2% per turn, 3% if you have democracy or star democracy)

do not terminate trading, but risk getting plagued themselves (20% per turn for getting it from merchandising and other reasons, maybe you could be able to increase some security measures and get "only" 10% per turn, but it would cost you some $$ and more, dunno)
I think it would ad a new interesting twist to the games with this event, post your comments here!

just had a thought... maybe if you are evil... you could... "purge" your merchants and "give big payments" to their families, so you have only 5% risk for getting plague while still doing the trading with plagued races... possibilities are endless...