Pyometra is a deadly infection that affects intact females of every breed including mixes.

http://www.marvistavet.com/html/body_pyometra.html is a good article on Pyometra. I know of females that have died from it. One of my own Champions almost died from developing closed Pyometra. It was only quick attention from her owners and a fabulous emergency vet that saved her life; just barely.

http://www.thepetcenter.com/sur/pyo.html is another good article, though I put a caution up for the graphic photos it includes. Once you look at these photos, you get a good idea just how serious this can be.