If you were to take a microscopic look inside your body, you’d find trillions of organisms living in just about every part of you, from the skin of your elbow to inside your mouth, nose and (yup) vagina. In fact, microscopic organisms outnumber your own cells 10-to-1. That amounts to about a pound or two of bacteria in you at all times.These masses of microbes are integral to keeping your systems working smoothly, especially when it comes to digestion. Most of these microorganisms reside in your digestive tract and work together to help you break down food and absorb nutrients.

“You couldn’t survive without them,” says Sharon Richter, R.D. “And you need them all because they work in conjunction with each other.” These microorganisms also play a supportive role in the immune system and create a protective barrier to help ward off invading pathogens from outside. Microscopic organisms outnumber your own cells 10-to-1. That amounts to about a pound or two of bacteria in you at all times.

But some things can throw off the delicate balance of your gut bacteria. Illness, diet, stress and antibiotics—which are meant to kill bad bacteria that make you sick, but may attack your good bacteria in the process—can tamper with the populations of the various species, decimating some and allowing others to multiply unchecked. And many people just naturally have trouble keeping their teeny tenants in proper proportion, which often results in frequent tummy troubles like diarrhea, constipation, bloating—or an uncomfortable combination of the three. That’s where probiotics come in.

Probiotic Basics: Probiotics are dietary supplements that consist of living bacteria or yeast to help replenish and re-balance the so-called microflora in your intestines. (They can also be found in dairy products such as yogurt, milk and cheese labeled as containing “active cultures.”) Two common probiotics you might have heard of, Lactobacillus acidophilus and bifidus, are found naturally in the small and large intestines, respectively, and can be taken regularly to keep the digestive system humming along.

The good bacteria in probiotics also support the immune system, vaginal and urinary health, and have been shown to be helpful for a number of far-ranging issues, including allergies, ulcers, eczema and maybe even high cholesterol. You can use them daily for microbial maintenance, or take a them as-needed, say, right before or right after a meal you just know is going to be trouble later, or when you’re on antibiotics. The number of probiotics on the market has increased tenfold in less than a decade

Picking a probiotic isn’t always easy: There are a ton of different bacteria and various strains of each, and they don’t all target the same issues. The number of products on the market has increased tenfold in less than a decade, so navigating the probiotics aisle can be gut-wrenching in its own right. The road to the right probiotic for you has as many twists and turns as your small intestine. Richter stresses the importance of talking to a representative at a health food store or supplement shop to discuss your particular symptoms, history and personal preferences.We asked The Vitamin Shoppe’s probiotic expert Eric Cohen to guide us through some of the most common uses for probiotics and how to find the best product for your needs:

What you’ve got: Diarrhea

What to look for: Saccharomyces boulardii is a probiotic yeast that can deter bacteria from adhering to the intestinal lining, preventing over-proliferation. It is hearty enough to survive splashes of stomach acid. Florastor, a popular supplement capsule, has yeast found naturally on the skins of some tropical fruits.Some probiotics require refrigeration to keep the microbes kicking. Since traveler’s diarrhea is common, Cohen recommends packing a probiotic that can be stored at room temperature when you’re away from home. He suggests The Vitamin Shoppe Ultimate 10 Probiotic, Dr. Ohhira’s Probiotic Original Formula, Jarro-Dophilus or UAS Labs Probioplus DDS.

What you’ve got: Yeast infection

What to look for: For a happy, well-balanced vagina, there’s Garden of Life’s Raw Probiotic for Women, ReNew Life Ultimate Flora Vaginal Support and Jarrow’s adorably-named Fem-Dophilus. These formulas offer additional benefits for the urinary tract.

What you’ve got: A prescription for antibiotics

What to look for: “Antibiotics can rapidly deplete the balance of friendly intestinal flora, which may result in digestive issues,” explains Cohen. He says that, in order to more quickly restore your microfloral friends, you should opt for products with a CFU of 50 billion or higher. CFU stands for colony-forming units, a measure of how potent the probiotic is.  Some examples: The Vitamin Shoppe Ultimate 10 Probiotic 50 Billion and Nature’s Way Primadophilus (90 billion or 100 billion). Florastor could come in handy here, too, since yeast is impervious to antibiotics, so it won’t be counteracted by your Rx.