Women tend to get cystitis more often than men because the female urethra (tube that brings urine from the bladder out of the body) is much shorter than in males and sits closer to the back passage compared to males.
This means that bacteria can get into the bladder via faeces (poo) more easily . E.Coli bacteria, from the bowel, is the most common cause of female cystitis.
Some things can increase your risk of developing cystitis:
having sex
wiping from back to front (instead of front to back) after going to the toilet
being older than 65 years (10% of women over this age report having a UTI in the last 12 months)
Pregnancy
using a diaphragm / spermicide & condoms for contraception
diabetes
a weak immune system