The vagina has multiple functions, including[4][6][4]:

• Menstruation- as a canal for menstrual fluid and tissue to leave the uterine cavity
• Immune defense- protection against harmful pathogens via acidic pH, local flora, and chemical signaling
• Reproduction functions- as the receptacle for sperm and canal for parturition
• Sexual functions - for female arousal response and sexual pleasure 

While the vagina does not contain glands, fluid transudates through the vaginal walls to create lubrication. The fluid is leaked from the capillaries due to vasocongestion into the space between vaginal epithelial cells and ultimately into the lumen.[7] This process is heavily dependent on the actions of nitric oxide (NO) in the endothelium, which gets released in response to sexual arousal due to stimulation of parasympathetic nitrergic nerves.[7] NO induces vasodilation through the cGMP cascade and decreased phosphodiesterase enzyme leading to smooth muscle relaxation. Major components of the vaginal secretions include water, desquamating epithelial cells, cervical mucus, fluids from the upper genital tract, and leukocytes. Estrogens and sexual stimulation are examples of factors which increase vaginal fluid. Estrogen enhances NO synthase function.[8] Major organic constituents of vaginal fluid are proteins, carbohydrates, and fatty acids.[9][10][11]

Vaginal pH testing, wet prep, potassium hydroxide preparation, and culture are frequently performed during a pelvic examination to help elucidate the cause of vaginitis.[12] A biopsy may be necessary at this location if a lesion is visible on speculum examination or clinical suspicion for malignancy. 

Changes in the bacterial flora and vaginal pH are closely related to vaginitis. The normal vaginal pH is from 3.8 to 4.5. This pH is secondary to acid-producing bacteria and helps to maintain the local flora, kill sperm, and produce immunologic protection against bacterial overgrowth.[12]

Candida is a naturally occurring organism that can proliferate and result in an overgrowth and yeast infection. Elevated blood sugars and COC pills increase the risk of yeast infection.[12][13]

The sexually transmitted parasites such trichomonas is acquired through sexual contact with an infected partner. This organism can be transmitted back and forth between partners unless both partners receive adequate treatment.[14]

HPV infection is sexually transmitted and highly associated with vaginal squamous cell carcinoma and related premalignant lesions, which occurs via proteins E6 and E7 interrupting proper DNA checkpoints leading to DNA mutations and nuclear changes. This situation can lead to dysplasia and eventual malignant neoplasia of the tissue.[15][16]

Vaginitis - Inflammation of the vagina can result from the overgrowth of local bacteria, yeast, or sexually transmitted infection. Bacterial Vaginosis (BV) characteristically presents with an alkaline pH and predominance of Gardnerella and Mycoplasma spp. BV is often associated with a ‘fish-like’ odor. Vaginal candidiasis is an overgrowth of a commensal organism and is another cause of vaginitis characterized by pruritus, thick ‘cottage cheese-like’ discharge and changes in vaginal flora. Trichomoniasis is a sexually transmitted disease caused by a parasite that is commonly associated with a clinical picture of yellow-green frothy discharge and vaginal irritation. Non-infectious vaginitis can be due to allergic reactions or irritation secondary to applying products to the vagina such as douches, scented perfumes, and soaps.[12][17]

Vaginal neoplasia - Vaginal intraepithelial neoplasia (VAIN) precursor lesions are associated with HPV and may progress to malignant vaginal squamous cell carcinoma; this is the most common form of vaginal malignancy. Vaginal clear cell carcinoma is associated with DES exposure in utero due, in part, by the persistence of columnar cells. Several more rare neoplasms, both malignant and benign can occur in the vagina.[15][16][18] 

• Benign vaginal lesions:  angiomyofibroblastoma, fibroepithelial polyp, hymenal polyp, leiomyoma, Mullerian papilloma, polyposis, vaginalis postoperative spindle cell nodule, pseudosarcomatous fibroepithelial stromal polyp, rhabdomyoma,  spindle cell epithelioma, squamous papilloma, superficial cervicovaginal myofibroblastoma, tubulovillous adenoma 
• Malignant vaginal lesions: adenocarcinoma, aggressive angiomyxoma, clear cell adenocarcinoma, endometrioid adenocarcinoma, granulocytic sarcoma leiomyosarcoma, lymphoma, malignant mixed tumor melanoma, mesonephric adenocarcinoma, metastases, mucinous adenocarcinoma, myeloid sarcoma, rhabdomyosarcoma, small cell carcinoma, spindle cell carcinoma, squamous cell carcinoma, urothelial carcinoma, VAIN, verrucous carcinoma, yolk sac tumor

Vaginal trauma - Causes range from vaginal intercourse to parturition and iatrogenically from instrumentation.  Tears can range in severity from mucosal involvement to underlying muscle involvement.[19] A rare postoperative complication is vaginal evisceration.[20]

Vaginal atrophy - Declining levels of estrogen during peri-menopausal and menopausal years are associated with thinning of the vaginal walls and loss of rugae.[21] This atrophy can cause pain, bleeding from micro-tears, and painful intercourse also called dyspareunia.[22][23]

Vaginal prolapse - Vaginal vault prolapse occurs with weakened vaginal walls, fascia, and ligaments that are supporting the surrounding structures such as uterus, rectum, bowel, and bladder. These may prolapse out of the vaginal canal and present as a bulge, sensation of increased pressure and a spectrum of changes in bowel and urinary continence.[24][25]