Strep throat is just the beginning of what the Streptococcus genus of bacteria can do to you. You can also develop meningitis (life-threatening inflammation of the brain and spinal cord), erysipelas (deep red skin lesions) or even necrotizing fasciitis (flesh-eating disease), depending on the strain. But Streptococcus' bad reputation isn't entirely deserved. Streptococcus thermophilus, for example, is one of the superstars behind yogurt and cheese (pdf). To make Swiss cheese, Streptococcus thermophilus is mixed with another bacterium, Lactobacillus bulgaricus, and milk. The bacterial duo are symbiotic: Lactobacillus breaks down milk proteins, which feed Streptococcus. Streptococcus, in turn, makes formic acid and carbon dioxide, which stimulate the growth of Lactobacillus.
Working together, the bacteria ferment the milk, converting lactose sugars into lactic acid. With the help of a few enzymes, and several steps of cooking, pressing and drying, the fermented milk turns into a thick custard, then curds, and finally delicious cheese.