These infections are so common that three-fourths of women will get one at some point. The hallmark symptom is extreme itchiness, along with an odorless thick, white discharge. Trichomoniasis This common sexually transmitted infection STI is caused by a parasite called Trichomonas vaginalis. But when it does, it causes itching, burning, a change in discharge, or external white cracking in the skin, Shantz-Dunn said. Irritation Rounding out the top three most common causes, Schantz-Dunn says, is irritation caused by certain fabrics or products.