Despite their bad-guy reputation, bacteria aren’t all evil. In fact, your digestive tract contains ten times more bacteria—the good kind—than all the rest of the body’s cells combined. Also called intestinal flora, these beneficial bugs help you digest food, keep your bowels regular, and make tiny amounts of some vitamins. “Probiotics should be considered indispensable essential nutrients,” says Marcus Laux, ND, of San Francisco.

For routine maintenance and to prevent illness, many integrative physicians recommend probiotics daily from supplements or food, such as active-culture yogurt. In particular, lactobacillus and bifidobacteria probiotics essentially function as microbial “anchors” in the gut, supporting populations of other good bacteria; supplement with 1 billion CFU lactobacillus and bifidobacteria species daily. (CFU stands for colony forming units, or the number of living and reproducing bacteria.)

Unfortunately, most people don’t think about probiotics until they’ve developed a problem. “Intestinal dysfunctions, whether from prescribed antibiotics or traveler’s diarrhea, demand probiotic supplementation to promote healing and to support a faster, full recovery,” Laux says. But these gut helpers can be confusing because there are so many different types. Here’s how to use friendly flora to keep your gut and its tiny residents happy.

Problem: antibiotic-induced diarrhea 

Antibiotics fight nasty bacterial infections, but they’re indiscriminate and kill off the good bacteria with the bad. That leaves some opportunistic germs, such as C. difficile, to fill the void, leading to diarrhea and setting the stage for a candida yeast infection. Without affecting antibiotics, probiotics will help you recover from diarrhea and start healing your gut.

Fix: As soon as you start an antibiotic, take probiotics at least once daily. The most helpful strains include Lactobacillus GG (a proprietary strain), L. rhamnosus, mixtures of multiple probiotics, and the probiotic yeast S. boulardii.

Dose: 1 billion to 5 billion CFU daily

Problem: infectious diarrhea

Numerous studies indicate that probiotics can prevent or resolve infectious diarrhea (for example, from food poisoning), especially in infants and children.

Fix: S. boulardii can yield rapid benefits, but it must be taken within 72 hours of the infection’s onset. Other helpful probiotics include B. lactis and S. thermophilus, which together can also reduce rotavirus infection risk. For small children, open a capsule and mix the contents into a cold drink.

Dose: 1 billion to 10 billion CFU daily

Problem: traveler’s diarrhea 

If you’re planning to visit developing countries, be proactive about your health. Start taking probiotics a couple of weeks ahead—and pack extra for the trip.

Fix: For prevention, use Lactobacillus GG. To recover from traveler’s diarrhea, your first choice should be S. boulardii; other helpful strains include L. acidophilus, L. bulgaricus, and S. thermophilus.

Dose: 50 billion to 100 billion CFU daily

Problem: vaginal and candida infections

Probiotics can help heal these common (and annoying) maladies, as well as resolve urinary tract infections in women, because probiotic bacteria secrete their own “antibiotics,” enabling them to successfully compete against disease-causing germs. 

Take: For bacterial vaginosis, try L. acidophilus, L. rhamnosus, and L. fermentum. For a yeast infection, try S. boulardii, ideally with one or more of L. acidophilus, L. rhamnosus, B. longum, and B. bifidum; start at a low dose and increase over two weeks to avoid symptoms of a rapid yeast die-off.

Dose: 1 billion to 10 billion CFU daily

How to store probiotics

Always take probiotics right after eating a meal, and store them in the fridge or a cool, dark place. 

Remember: it’s a good idea to talk to your health care provider before starting a new supplement.