For centuries, stories of priests finding a red color in communion wafers have been archived and actively propagated by the religious folks. As it turned out eventually, the red in those wafers was due to a particular bacteria.
The bacteria responsible for the mischief is Serratia marcescens and it has been known for centuries. Initially, scientists considered it a benign bacteria which didn’t wreck any significant damage and decayed only the dead tissues.
However, Serratia marcescens was found to be the culprit in numerous cases of serious hospital infections in 20th century. This finally revealed that the bacteria was not as benign as it was originally believed to be. Not only did it cause many infections, the bacteria was also found to be the reason why a red tinting can be found in some communion wafers.
As Catholic beliefs go, communion wafers are a symbol of the body of the Christ. So when the communion wafers would turn red, thanks to Serratia marcescens, people would start thinking that the blood of Christ had appeared on the wafers and thus, there beliefs would be reaffirmed. It was only in 1819 that a pharmacist decided to investigate the issue and found that the red tinting was simply the color of the bacteria inhabiting bread. The pharmacist who is credited was finding this out is named Bartolomeo Bizio.