The good bacteria in your gut

It wasn't until recently that doctors and scientists knew how important the good bacteria in your gut are.  Gut microbiota live in the digestive tract of animals, and help them digest food as well as absorb vitamins. One study from UCLA shows that eating probiotics via yogurt can change how a person's brain works. There are many sources of probiotics in pill form but they are useless, the bacteria simply don't survive the drying out process.

From Wikipedia:

Gut microorganisms benefit the host by collecting the energy from the fermentation of undigested carbohydrates and the subsequent absorption of short-chain fatty acids. The most important of these fatty acids are butyrates, metabolised by the colonic epithelium; propionates by the liver; and acetates by the muscle tissue. Intestinal bacteria also play a role in synthesizing vitamin B and vitamin K as well as metabolizing bile acids, sterols and xenobiotics. 99% of the bacteria in the gut come from 30-40 species. Bacteria make up 60% of the dry weight of feces. The currently known genera of fungi of the gut flora include Candida, Saccharomyces, Aspergillus, and Penicillium.

70% of Upper Respiratory Infections are from viruses, not bacteria, so antibiotics will not help you most of the time. Overuse of antibiotics can lead to an imbalance of the gut bacteria, allowing bad bacteria to take over. More about gut flora and antibiotics:

Altering the numbers of gut bacteria, for example by taking broad-spectrum antibiotics, may affect the host's health and ability to digest food.^[52] Antibiotics can cause antibiotic-associated diarrhea (AAD) by irritating the bowel directly, changing the levels of gut flora, or allowing pathogenic bacteria to grow.

Fiber is said to encourage the growth of helpful gut microbiota, but you must eat it every day in enough quantity to be helpful.

Bacteria and cancer:

Some genera of bacteria, such as Bacteroides and Clostridium, have been associated with an increase in tumor growth rate, while other genera, such as Lactobacillus and Bifidobacteria, are known to prevent tumor formation.

Gut bacteria and obesity:

The mutual influence of gut flora composition and weight condition is connected to differences in the energy-reabsorbing potential of different ratios of Firmicutes and Bacteroidetes, especially in the digestion of fatty acids and dietary polysaccharides, as shown by experiments^[63] wherein the (caecum) gut flora of obese mice were transplanted into germ-free recipient mice, leading to an increase in weight despite a decrease in food consumption. The experimentation and results suggest bacteria specific to lean and obese genes can be a weapon in the fight against the obesity epidemic.

Fecal bacteriotherapy (or "poop pill") is an attempt to give a person with the wrong ratio of bacteria, a healthy set of bacteria. But so far the bacteria do not survive drying out, and a "fresh" pill is not so appetizing.

So be kind to your bacteria, do not take antibiotics unless you really have to, and eat more fiber to encourage the growth of good bacteria.