If the vaginal flora is compromised, the protective role of the lactobacilli (the good bacteria) is reduced. Due to reduced numbers of lactobacilli, less lactic acid is produced, and the protective low vaginal pH is affected. As a result, bad bacteria take the opportunity to overgrow.
There are many factors that can disturb the delicate balance that exists inside the vagina. Some of them include tissue damage caused by giving birth; health issues, stress, certain medication, tight clothing or synthetic underwear; inappropriate hygiene; irritants present in lotions, body washes and other cosmetic products; friction caused by penetration if the woman is not sufficiently aroused during intercourse; hormonal changes, etc.
During pregnancy, the amount and diversity of the vaginal lactobacilli is often reduced, putting pregnant women at a higher risk of an infection.