If you have been taking antibiotics (the kill both the good and bad bacteria), eating a high carbohydrate diet, experience flatulence and indigestion or get sick quite frequently, you very possibly have an imbalance in your bowel flora (the good and bad bacteria that live in your gut). Getting this right again is vitally important to maintaining optimum health as the good bacteria (probiotics) actually produce many enzymes, natural antibiotics, and other substances that benefit our health.
If you do decide to take some action on this, the best types of supplements to take are acidophilus and bifidobacterium supplements. Eating yogurts and fermented dairy and other products can also be beneficial, but because of many of the manufacturing processes involved these days, the potency of the probiotics in the products are generally not enough to be used therapeutically. Also, finding a supplement that has been refrigerated throughout the process in order to keep the bacteria viable is very important. Otherwise what is on the label, isn't necessarily what you will be getting.