STD=sexually transmitted diseases... you need to have sex to get it. Or heavy petting of genital areas which might get you herpes/warts
for other bacteria/virus, well its everywhere actually.. incl the keyboard that you gonna use to reply this. (little known that PC keyboard is actually very very dirty)
widely known that sharing of towel is not recommended anyway, ..as you may transmit certain everyday common viruses that causes flu and fever.. and more rarely even skin infection.
For that reason, good hotel clean their towel via chemical detergent and then dry it using high heat dryer.
however, these issue shld not concern you as its just part of life.. just use common sense (like wash ur hands after peepee etc.).. and if you still have worries of say, towel usage in a hotel, then probably u hv Bacteriophobia..