There are several ways to get an infection:
By disturbing the natural environment. Things you should not do: wearing nylon underpants and/or tight jeans, washing your vulva with soap, using feminine hygiene sprays, taking frequent bubble baths and vaginal douching.
Wiping your bottom from the top downwards, causing faeces to come near the vaginal opening.
Taking antibiotics or the contraceptive pill.
Having intercourse too frequently.
Eating too much sugar might also be of influence.
From your lover, if he has an infection (sometimes the guy has no symptoms) and you have intercourse without using a condom.

See also: ping pong effect