We human beings carry several times more bacteria around with us than actual body cells. Some help us to digest food, others to stave off nastier microorganisms. Certain staphylococci belong to the normal flora of human skin—just about all of us are colonized by Staphylococcus epidermidis, a bacterial species with round cells that like to cluster. It is also the dominant Staphylococcus species in the nose, and recent research suggests it keeps its not-so-friendly sibling Staphylococcus aureus within bounds there.

The species Staphylococcus aureus is a major cause of food poisoning and dangerous infections worldwide, particularly among people with a weak immune system. So don’t be fooled by its good looks—golden yellow colonies, hence "aureus" ("gold" in Latin). Some strains, known as MRSA, are a huge health concern because they have become resistant to the antibiotics we use. Nevertheless, about 20 percent of all people carry S. aureus permanently in their noses, and another 20 percent occasionally. Its close relative S. epidermis seems to prevent it from causing mischief there. 

Corynebacterium are very irregularly shaped bacteria. Some look like the letter V under the microscope, others like a club (hence the name: coryneform means "club-shaped"). The species C. diphtheriae and C. ulcerans can cause diphtheria, a potentially deadly disease if the bacterial cells are themselves infected by a certain virus that prompts them to produce a toxin. Pet dogs can carry corynebacteria into the house, so washing your hands is a good idea—as is, of course, to regularly get your booster injections that should keep you immune.

To get the energy they need, some bacteria rely entirely on fermentation. In fact, many of these cannot live or grow when oxygen, or too much of it, is present—for example some potentially pathogenic fusobacteria species that populate the mouth and nose of humans. You wonder how these anaerobic bacteria can survive in a place where air passes through all the time? In normal times they subsist in low-oxygen pockets or within protective films of slime (biofilms) alongside other microbes, hoping to make it big time some day. Fame and fortune come when, for example, an abscess forms inside of which they can thrive.

The genus Haemophilus contains a number of human and animal pathogens, most notably H. influenzae. This bacterium got its name because it was believed to cause influenza before viruses were discovered—in truth it merely exploits the weakness of influenza patients for its own benefit. What H. influenzae does cause is sepsis and bacterial meningitis in young children. Well, not all strains do. The nice strains are actually a natural part of our skin flora. The nasty ones, however, encapsulate themselves to prevent immune cells from removing them. Up to the year 2000, H. influenzae killed hundreds of thousands of small children every year around the globe. An effective vaccine has since reduced the fatalities by around 90% where it is widely available.