Strep throat is caused by one species of bacteria, Streptococcus pyogenes, but that single species contains more than 275 genetically distinct strains. So the answer depends on what you mean by “types.” There is essentially one disease called strep throat, caused by one bacterial group, but the organism behind it is far more diverse than most people realize.
One Bacterial Group, Hundreds of Strains
Strep throat is an infection of the throat and tonsils caused by group A Streptococcus, also known as Streptococcus pyogenes. In the classification system microbiologists use to sort strep bacteria (the Lancefield system), this organism falls into group A based on a specific molecule on its surface. When doctors talk about “strep throat,” they are referring to pharyngitis caused by this single bacterial group.
However, within group A Streptococcus, scientists have identified more than 275 distinct strains using a genetic typing method called emm typing, which looks at variations in a key surface protein. These strains differ in how they interact with the immune system, which is one reason you can get strep throat more than once. Your body builds immunity to the specific strain that infected you, but dozens of other strains can still cause a new infection. This strain diversity is also why developing a universal strep vaccine has been so difficult.
Other Strep Groups That Affect the Throat
Group A strep is responsible for the vast majority of bacterial throat infections, but it is not the only streptococcal group that can cause a sore throat. Group C and group G streptococci are normal inhabitants of the human upper airway and usually live there without causing problems. Occasionally, though, they cause pharyngitis that looks clinically identical to a group A infection.
These non-group-A infections are far less common and generally considered less dangerous. They are not typically associated with the serious complications that can follow a group A infection, such as rheumatic fever. Standard rapid strep tests only detect group A strep, so a group C or G infection would show up as a negative result on the quick test, though a throat culture could identify it.
Strep Throat vs. Scarlet Fever
Some people wonder whether scarlet fever counts as a separate type of strep throat. It does not. Scarlet fever is strep throat caused by a strain of group A strep that produces a particular toxin, which triggers a distinctive red, sandpaper-like rash across the body. The throat infection itself is the same. The rash is simply an additional reaction to that toxin. Both strep throat and scarlet fever are treated the same way, with the same antibiotics, and both resolve on a similar timeline.
Carriers Without Symptoms
Not everyone who harbors group A strep in their throat is actually sick. Some people, particularly children, are asymptomatic carriers. In a typical pediatric practice, roughly 2% to 4% of children carry the bacteria at any given time without symptoms. In school classroom surveys, that number climbs to 15% to 20%.
Carriers are not considered to have strep throat. They are unlikely to spread the bacteria to others and generally do not need treatment. This matters because a child who carries the bacteria can test positive on a rapid strep test even when their sore throat is actually caused by a virus. This is one reason doctors look at the full picture of symptoms rather than relying on a test result alone.
How Doctors Distinguish Strep From Viral Sore Throats
Since most sore throats are viral, clinicians use a scoring system (the Centor score) to estimate the likelihood that a sore throat is bacterial before testing. The criteria include fever, swollen and tender lymph nodes in the front of the neck, white or yellow patches on the tonsils, and the absence of a cough. A cough, runny nose, or hoarseness typically point toward a virus instead.
No single symptom is reliable enough to diagnose strep throat on its own. In adults, clinical symptoms alone are only moderately useful for ruling strep in or out. In children ages 3 to 17, a high score on the modified version of this system raises the probability of strep significantly, with a likelihood ratio of about 4.0. But the scoring system works best when paired with a rapid strep test. For adults with a high symptom score and a positive rapid test, the probability of a true strep infection is extremely high.
Complications From Untreated Infections
One reason it matters to identify and treat strep throat is the risk of complications from untreated group A infections. Rheumatic fever, which can damage the heart valves, is the most well-known. It occurs when the immune system’s response to the strep bacteria mistakenly attacks the body’s own tissues, particularly in the heart and joints.
A less common but increasingly recognized condition is PANDAS, which stands for pediatric autoimmune neuropsychiatric disorders associated with streptococcal infections. In PANDAS, the immune system’s antibodies against strep bacteria are thought to cross-react with cells in the brain, triggering sudden-onset obsessive-compulsive behaviors, tics, or anxiety in children. Children who have frequent group A strep infections may be at higher risk. PANDAS is still an area of active debate among specialists, but it illustrates how a single bacterial species can produce a wide range of effects beyond a sore throat.
These complications are not separate “types” of strep throat. They are downstream consequences of the same group A strep infection when it is not adequately treated or when the immune response goes awry.