Yogurt has the same good bacteria in it that is found in your vagina...when you have an infection it is usually due to an imbalance in the bacteria in your vagina. If you have an infection such as BV very common in women you are producing too much bad bacteria along with if you have a UTI or yeast infection also very common so when you eat yogurt you are helping ballance your natural ph
Yogurt is cool stuff. Taken orally it naturally helps bacteria in your stomach and intestines balance out to healthy amounts. It is also great for any type of yeast infection (vaginal, athlete's foot depending on what kind, etc.) and should also be taken after a round of antibiotics since those wreck your internal bacteria, both good and bad.
You can also take acidophilus by pill. Acidophilus is the good bacteria that's found in yogurt. Whatever you do, make sure if you're using yogurt, make sure it is the plain yogurt. Sugar FEEDS yeast so if you have an infection stay away from sugar until you get it under control. I worked in the health food (alternative medicine) industry for 5 years and time and time again this worked.
A quick glance through wikipedia's page on yogurt would suggest that all of the bacterial cultures in yogurt are beneficial (in slightly different ways) but help your body either through digestion or some form of disease protection/prevention. I personally love yogurt, definitely great stuff.
The science fact: Lactobacillus are found in plain yogourt. We actually have lactobacillus there for protection usually, they produce lactic acid which is acid (you don't say) and protects against microorganisms. When those bacterias die, the opportunist, Candida albicans, takes over. Yeast chaos. But use plain yogourt. Because Candida albicans LOVES sugar.
I've found plain yogurt applied externally works well to relieve itching but hasn't done much to actually relieve the infection. A garlic clove applied in the vagina works, it's stinky but effective. Gentian violet is also supposed to work but I'm wary of it (and having a bright purple cooch for a few days). Monistat burns like the fire of 1,000 suns to me so I've tried many more natural methods.
Eating plain yogert can help, but it needs to have active cultures like Activia or Stoneyfield farms. Most of what is sold in stores is pasturized and all the good bacteria has been killed.
It needs to be plain because most yogert has a ton of sugar and yeast feeds off sugar, so if you eat a sugary yogert you will just be making the yeast stronger and the yeast will kill all the good bacteria the yogert contains.