Most UTIs are due to bacteria that are normally found in your gut, such as E. coli. Other bacteria that can cause them include staphylococcus, proteus, klebsiella, enterococcus, and pseudomonas.
Some bladder infections in both men and women are linked to sexually transmitted infections, including Chlamydia trachomatis, mycoplasma, and ureaplasma. The parasite trichomonas can also cause similar symptoms.
If you have a problem with your immune system, from an autoimmune disease or diabetes, you're more likely to get UTIs because your body can't do a good job fighting off the germs that cause these infections.