What Is Vaginal Odor?
Vaginal odor is the appearance of a malodor from the vagina with or without the presence of malodorous or normal vaginal discharge.
Vaginal Odor: Causes
The most common cause of vaginal odor is an overgrowth of the mucous lining of the vagina with anaerobic bacteria, which is called bacterial vaginosis or BV. In a healthy vagina, there are many bacteria that live on the mucous membranes. These bacteria are usually lactobacilli similar to the ones that ferment milk into yoghurt. These bacteria are called the natural vaginal flora and they help to keep the pH of the vagina low, which in turn helps stop the growth of other unwanted bacteria and also of certain fungi. 
Nevertheless, overgrowth of the vaginal mucous membranes with other, unwanted, bacteria can happen. Most frequently, these bacteria are a mixture of different kinds, with Prevotella sp. and Mobiluncus sp., G. vaginalis, and Mycoplasma hominis being the most common ones. It is not completely clear why this bacterial overgrowth happens, but the risk of developing this condition is higher with multiple sexual partners, a new sexual partner, douching, uncontrolled diabetes mellitus, and the lack of vaginal lactobacilli. 
Douching is particularly common practice in women although it is know that it is harmful and should be avoided because it can cause PID (pelvic inflammatory disease), ectopic pregnancy, bacterial vaginosis (BV), cervical cancer, low birth weight, preterm birth, human immunodeficiency virus transmission, sexually transmitted diseases, recurrent vulvovaginal candidiasis, and infertility. 
The lack of vaginal lactobacilli can happen (for example) after a different condition in the body was treated with antibiotics that were taken either as pills or as an injection, as antibiotics will kill bacteria all over the body without discrimination.
Whether BV is a sexually transmitted disease is not clear, as the treatment of the sexual partner does not help in preventing another outbreak in women, but women who were never sexually active rarely ever develop this condition.
Another common reason for vaginal odor or a malodorous vaginal discharge is an infection with the protozoan Trichomonia vaginalis.
A protozoan is not a bacterium or fungus, but a much bigger microbe. Many protozoans can cause human diseases like malaria, and different kinds of diarrhea (caused e.g. by Guardia sp. or amoebas).
An infection with T. vaginalis is called Trichomoniasis. It is a sexually transmitted disease (STD) that can cause urethritis or the inflammation of the urethra in men. In women, a malodorous, yellow-greenish vaginal discharge and irritation of the vulva is often associated with this infection. Discomfort during sex, painful urination, itching in or near the vagina is also a sign of trichomoniasis. 
Yeast infections, often caused by yeast of the genus Candida, also known as Vaginal Thrush or Vulvovaginal Candidiasis (VVC), can also cause vaginal malodor.
Symptoms like itching, vaginal soreness, pain during sexual intercourse, and external pain when urinating can mean that a yeast infection is the cause of the malodor, but similar symptoms can also be caused by other conditions. A white, cottage cheese-like vaginal discharge is typical for yeast infection.
Vaginal thrush is a very common condition, as it is estimated that about 75% of all women have at least one episode of it during their lifetime.  It is not considered an STD, but due to injuries to the vaginal lining that it can cause, it can increase the risk to contract an STD when a vaginal Candida-infection is present.
Vaginal Odor Prevention
Sexually transmitted vaginal infections like Trichomoniasis can be prevented by safe sexual practices, in particular, by the use of condoms. Laboratory tests can prove infection with trichomoniasis. If you are infected, you and your partner must be treated with antibiotics. 
Yeast infections are especially common in people who wear tight fitting clothes like tight jeans and/or panty hose that allow heat and humidity to build up. Wearing loose clothes and avoiding tight pants can, therefore, help to prevent yeast infections. 
Killing of the natural flora of lactobacilli in the vagina creates space for other not so friendly microbes like anaerobic bacteria and yeast to grow. Therefore avoiding things that can kill the friendly vaginal bacteria can also help prevent a bacterial or yeast infection. Among these things are the use of feminine hygiene products with lots of perfume in them, antibiotic treatments, frequent use of hot tubs (the chlorine in the water kills the friendly bacteria), and douching, a practice that is particularly common among African-American women. 
Many women douche because they think it might help prevent vaginal diseases, but numerous studies showed that the opposite is the case: douching can irritate the vagina, making it easier for microbes to attach to and infect the sensitive lining and it displaces or kills the natural bacterial flora that helps to prevent the overgrowth with other bacteria and fungi. 
Eating a well-balanced diet that contains yogurt with active culture can also help prevent vaginal infections. In diabetic women, it has also been suggested that yogurt consumption reduces the risk of vaginal yeast infection, caused by Candida, by regulating pH and suppressing Candida overgrowth. 
If you go swimming, change out of the wet clothes as soon as possible.
If your blood sugar is very high because you have uncontrolled diabetes, excess sugar will show up in the vagina and urine. This can make it a very favorable environment for microbes to grow in, as they will use the sugar as fuel. Therefore, if you have diabetes, keeping your blood sugar well controlled will help prevent vaginal infections. 
Washing underwear in hot water and/or with bleach will kill any microbes that sit in the underwear. This might help prevent the reccurrence of vaginal infections.
Vaginal Odor Treatment
If the vaginal odor is caused by bacterial infections, the main treatment is antibiotics.
The most commonly prescribed antibiotics for bacterial vaginosis are Metronidazole, which can be taken as a pill usually twice a day for seven days or as a vaginal gel that is taken once a day for five days, and Clindamycin, that is used intravaginally as a cream for seven days in a row.
Pregnant women need to be treated, as BV can cause severe complications, but they cannot be treated with vaginal creams or gels.
Metronidazole works also against Trichomonisasis, which can also be treated by another antifungal called Tinidazole.
In the case of Trichomoniasis, the sexual partner needs to be treated too, as otherwise a re-infection from the most likely also infected sexual partner is hard to prevent. Studies have shown that the treatment of the sexual partner in the case of BV does not decrease the chances of a reccurence.
A variety of intravaginally applied anti-fungal agents are available for the treatment of yeast infections. If a woman has uncontrolled diabetes or uses corticosteroids for other conditions, she might not respond well to these treatments. It should, therefore, be attempted to correct the other conditions simultaneously with an antifungal treatment for a vaginal yeast infection.