The reason strep throat is contagious is because GAS bacteria live in your throat and nose making it easily for them to spread to other uninfected people around you. The bacteria travel through small droplets of water, which are released out when you sneeze or cough. As the droplets are sneezed out, they carry the infectious bacteria and land on any surface they find. If you touch any surface which has the bacterial droplets on them and then touch your nose or mouth with your contaminated hands, the infection then starts spreading in you. Similarly, if you share a glass or plate with the infected person or shake their hands, there is a high chance that you may become infected as well. It is best to wash your hands after such occasionsfollowing casual contacts. As mentioned earlier, the bacteria GAS is responsible for causing strep throat, but the Streptococcus class of bacteria also causes impetigo, an infection which causes red sores all over the body, rheumatic fever in addition to some more. The bacteria start growing in the throat or skin which leads to pus formation and non-inflammatory continuation of previous infection. Impetigo is contagious while rheumatic fever is not contagious however, strep throat can come before it, which certainly is contagious. At some times, if you touch skin sores caused by GAS, like impetigo, you can become infected easily. It is again recommended that you wash your hands. Sometimes you can become infected from food that contains the GAS bacteria. Although it does not happen often, strep throat is foodborne as well. That is why before pasteurization, preservation and refrigeration of food was not common, strep throat spread quite commonly. In the past, outbreaks of strep throat used to happen quite commonly due to contamination and consumption of raw milk. But now foodborne strep throat is rare so if many people complain of strep throat after eating food together in a venue, it can indicate contaminated food. When it comes to strep throat, the bacteria starts infecting by colonizing in the throat. The bacteria adheres to the dermal epithelial cells which is due to a specific bacterial surface ligand binding with receptors in the throat. The strong adherence is one of the reasons strep throat cannot be simply washed away so easily. In fact, salivary fluid and mucous passing down the throat cannot scrape the bacteria strongly adhered to the skin of the throat so they continue damaging the tissues in the throat. Previous infections are also crucial as any exposure before leads to weakening of the dermal barriers so when strep throat happens again, the bacteria are able to adhere to the host cells quite easily. Studies have also shown that at certain sites, the other bacteria are in competition with the infectious cells for adherence to the host cells. Reference: http://bit.ly/2h220pC