I came across this old NPR article:
This is about what can grow inside your horn if you do not clean it regularly. It can make you seriously ill as it did this one trombone player in the story. Interesting read.
Of course Starving Students Music Supplies has an answer to this problem! H.W. Products has an item called the Brass-Saver. It is a set of instrument specific brushes used to clean out your horn. Check it out
BTW the picture above was what was living inside the trombone player's horn - Nasty stuff