If you buy yogurt, then you’ve likely noticed the recent appearance of yogurt packages saying they contain probiotics that can help your digestion or improve the functioning of your immune system. If you pay attention to business news, you may have heard about a lawsuit filed in January by a California woman asserting that Dannon’s probiotics advertising is making false health claims.

The National Institutes of Health website has a page on probiotics stating there is “encouraging evidence’’ that they may be useful in treating diarrhea, irritable bowel syndrome, atopic dermatitis, bladder cancer and other diseases.

And you’re scratching your head wondering just what a probiotic is.

To answer that and other questions about this emerging medical topic, we spoke to Gary Huffnagle, professor of internal medicine and microbiology/immunology at the University of Michigan Medical School and author of “The Probiotics Revolution’’ (Bantam Dell, $24).

What are probiotics?

Live microorganisms — almost always bacteria — that when ingested can confer a health benefit. They can be ingested in food, beverages or supplements. There is also a population of probiotic bacteria that naturally lives inside us, and those were ingested when we were infants. You can make the numbers of those go up or down by the food you eat.

Why should people care about these internal bacteria?

What we’re learning now, at an extremely furious pace, is that the bacteria that live inside of our body normally function together like an organ. They’re that important for our health. There are 100 trillion bacteria that live inside us. That’s 10 times more bacterial cells than our cells, so someone joked that we’re a minority in our body. This collection of microbes has a very important impact on our digestion, on how our GI tract works, our metabolism, and how our immune system works.

How can bacteria affect our immune system?

The most important way that bacteria affect our immune system is by providing chemical signals that make it slow down once an immune response has been generated. Normally, when you think of an immune response, you think about getting an infection, and then the immune response revs up. But once the infection is gone, the immune response keeps going unless something stops it. That’s how you get this widespread problem of chronic inflammatory disease. The microflora don’t play much of any role in what makes the immune system start, but they do play a role in what makes it slow down.

How do these friendly bacteria slow down the immune system?

The immune system is a system of white cells throughout the body. They talk to each other through chemical messengers. We’re learning that the microflora also makes some chemical messengers, and those chemical messengers are important signals to the immune system to slow down. The other thing probiotics can do is there are other bacteria that live inside us that can make chemical messengers, but they are bad signals. Probiotics elbow out the bad bacteria.

Why have probiotics become a big deal now?

The most widespread right now are the ones that are being marketed by Dannon. DanActive is one of them that contains a probiotic that has been shown in clinical trials to improve immune function in some people. It’s called Lactobacillus casei.

Then there’s Activia, which are the little yogurts. They contain a different probiotic bacteria called Bifidobacterium animalis.

Stonyfield yogurt is another probiotic. They don’t advertise it as a probiotic, but it contains Lactobacillus reuteri. All of these are actually good if you have any problems with diarrhea, especially following a bout of the stomach flu.