Like staphylococci, there are many species of streptococci found normally in the human body. Certain strains of streptococci cause some of the most serious diseases in humans. Streptococcus pyogenes (strep group A) is the main cause of bacterial pharyngitis (strep throat) in people.Untreated strep throat can lead to serious sequelae like rheumatic fever (heart valves) and glomerulonephritis (kidneys).hen bacteria is on the human body in the absence of disease, it is called a colonizer. However, people can get infected from pathogenic bacteria from food, water, abrasions and other wounds and even from colonizing bacteria if it gets into a normally sterile part of the body.