Species commonly found in humans: Klebsiella pneumoniae, Klebsiella oxytoca, Klebsiella granulomatis (all potential pathogens).

What it does: Klebsiella bacteria are generally found in human intestines, where they generally exist peacefully. However different types of this bacteria can spread and cause infection in sick patients in healthcare settings, including pneumonia, blood infections, skin infections, and meningitis.

Where it lives: Skin, nose, mouth, intestines, vagina.

Species commonly found in humans: Enterococcus faecalis (potential pathogen).

What it does: Enterococcus bacteria typically live harmoniously in the human intestines and female genital tract, but are also the leading causes of blood, surgical wound, and urinary tract infections.

Where it lives: Throat, mouth, intestines, urethra, vagina.

Species commonly found in humans: Escherichia coli (potential pathogen).

What it does: E. coli are a large and diverse family of bacteria that normally live in the intestines of people and animals, in the environment, and in certain foods. While most strains won’t make you sick, some can cause diarrhea, urinary tract infections, respiratory illnesses, and pneumonia.

Where it lives: Eye, nose, throat, mouth, intestines, urethra, vagina.

Species commonly found in humans: Haemophilus influenzae (potential pathogen).

What it does: This bacteria was mistakenly believed to be the culprit behind flu outbreaks back when it was first discovered in 1892. While most strains don’t cause disease in humans, the bacteria can cause respiratory tract and heart valve infections and sexually transmitted chancre sores in those with weakened immune systems.

Where it lives: Eye, nose, throat, mouth.

Species commonly found in humans: Streptococcus mitis, Streptococcus salivarius, Streptococcus mutans (potential pathogen), Streptococcus pneumoniae (potential pathogen), Streptococcus pyogenes (potential pathogen).

What it does: Streptococcal bacteria range greatly in their potential to cause disease and how they are spread. Group A Streptococcus generally lives harmoniously in the throat or on the skin, but can cause mild illnesses such as strep throat and skin infections.

Group B strep infections tend to be more severe and are more common in older or sick adults. Group B infections are the leading cause of meningitis and blood infections in newborns.

Where it lives: Skin, eye, nose, throat, mouth, intestines, vagina.

Species commonly found in humans: Neisseria gonorrhoeae, Neisseria meningitidis, Neisseria lactamica, Neisseria cinerea, Neisseria polysaccharea, Neisseria mucosa, Neisseria flavescens, Neisseria sicca, Neisseria subflava, Neisseria elongata, Neisseria gonorrhoeae (potential pathogen), Neisseria meningitidis (potential pathogen).

What it does: Of the several species of neisseria bacteria that live in humans, only two cause disease. These types are most notoriously known for causing meningitis and gonorrhoeae, which thrive in mucous membranes and are spread through sexual contact. Neisseria generally live in the upper respiratory tract and are not harmful to humans.

Where it lives: Eye, nose, throat, mouth, urethra, vagina.

Species commonly found in humans: Bacteroides caccae, Bacteroides distasonis, Bacteroides eggerthii, Bacteroides fragilis, Bacteroides merdae, Bacteroides ovatus, Bacteroides stercoris, Bacteroides thetaiotaomicron, Bacteroides uniformis, Bacteriodes vulgatus (all potential pathogens).

What it does: This bacteria has a complicated relationship with humans. When they’re isolated to the gut, they assist in breaking down food and synthesizing nutrients and energy for the body to use. But when they escape the intestines, they can cause particularly deadly infections in the blood and can form abscesses all over the body.

Where it lives: Intestines, urethra, vagina.

Species commonly found in humans: Clostridium perfringens (potential pathogen), Clostridium difficile (potential pathogen), Clostridium tetani (potential pathogen; is only transiently associated with humans, does not colonize the intestines).

What it does: This bacteria is commonly found in the soil and human intestines, and generally don’t cause issues. But a few strains of clostridium can produce potent toxins, including botulism, tetanus, and an irritation of the intestines. They can also cause a mild to life-threatening illness called Clostridium difficile, which causes inflammation of the intestines.

Where it lives: Mouth, intestines.

Species commonly found in humans: Several, some of which are potential pathogens.

What it does: This bacteria is most notorious for causing severe illnesses such as tuberculosis, leprosy, and Hansen’s disease, though most species of mycobacteria in nature are benign in humans, unless in cases of those who have weakened immune systems.

Where it lives: Skin, nose, throat, intestines, urethra.

Species commonly found in humans: Pseudomonas aeruginosa (potential pathogen).

What it does: This microbe is extremely versatile and can live in a wide range of environments, including soil, water, animals, plants, sewage, and hospitals in addition to humans. It seldomly sickens healthy people, but more typically causes blood infections and pneumonia in those who are hospitalized or have weakened immune systems.

Where it lives: Throat, mouth, intestines, urethra, vagina.

Species commonly found in humans: Several, some of which are potential pathogens.

What it does: The role of this bacteria, which is most well-known for causing syphilis and Lyme disease, is still not well-understood in humans. We’ve known for decades that it lives in the gut, but whether or not it’s a beneficial or pathogenic organism is still up for debate.

Where it lives: Throat, mouth, intestines.