Source: NewYorker.com

Bacteria=bad, right?  Well, not always, and perhaps not usually.

The abstract for this story gives one a taste for the discoveries within it:

Human beings are inhabited by as many as ten thousand bacterial species; the bacteria living inside us weigh, all told, about three pounds (the same as our brain). These bacteria work constantly on our behalf: they manufacture vitamins and patrol our guts to prevent infections; they help to form and bolster our immune systems, and digest food. Recent research suggests that they may even alter our brain chemistry, thus affecting our moods and behavior; when certain kinds of bacteria are destroyed, conditions like asthma and obesity, may result.

Read more at http://www.newyorker.com/reporting/2012/10/22/121022fa_fact_specter