Serratia marcescens is a bacteria that has earned a bad reputation for infecting people in hospitals. It may deserve an even worse reputation. It might have made people believe, for hundreds of years, that the blood of Christ was miraculously appearing in communion wafers.
Serratia marcescens used to be the workhorse of the biology laboratory. It was thought to be benign and saprophytic - scavenging only on dead or decaying tissue. Researchers used to put their hands in huge colonies of it and shake hands, then see how much bacteria had been transferred from one hand to another. In the latter half of the 20th century, people noticed that it was commonly involved in serious hospital infections. In 2004, it contaminated a company's entire stock of flu vaccines. The contamination was discovered early and the vaccines were never shipped, but there was a shortage of flu vaccines.