But don't worry, Ebola wasn't one of them.
New research into the health of New York City rats has revealed that they're carrying at least 18 previously unidentified diseases - as well as other known, pretty nasty illnesses.
The study, led by Columbia University's Mailman School of Public Health in New York, identified pathogens such as E. coli, Salmonella and the closest relative to human hepatitis C on record, living in rats. The results are published in the journal mBio.
Disease can easily spread when people come in contact with rat saliva, urine or feaces - something that happens quite frequently in rodent-infested cities such as New York. In fact, it's this kind of interaction between humans and rats that caused the "black death", or bubonic plague, of the 14th century.
To work out what kind of diseases the rats of NYC were carrying, the scientists trapped 133 rats at five sites around the city, focussing on those inside residential buildings in particular, for obvious disease-spreading reasons.
They then used molecular testing to look for known bacterial pathogens and viruses in the rats' tissue and excretions. They found that 15 of the 20 bacterial pathogens they were testing for were present in the rats, as well as one virus, Seoul hantavirus, which causes Ebola-like heamorrhagic fever in humans. This is the first time the virus has been documented in New York City, and the genetic clues in the rats suggest it's a new arrival.
Perhaps even more interestingly (and worryingly), the researchers also found 18 completely new viruses in the rats. None of these have been seen in humans as yet, but the scientists say that transmission is possible.
"While there's no immediate cause for alarm, the scientists note that that the spread of these new viruses from rats to humans could theoretically already be occurring and is possible in the future, and are advocating for more comprehensive disease monitoring in humans."
"It started as a biodefense initiative," said Ian Lipkin, senior author, about the study, in a press release. "If we are to pick up something that is a novel threat to public health, we have to know the baseline microflora."
The good news is that two of these new viruses are structurally very similar to the human Hepatitis C, which suggests that scientists could use them as animals models to research cures and treatments for the human virus.
This research will also help scientists keep an eye on any potential epidemics. As the press release explains: "With modern disease surveillance methods, a repeat of the rat-borne Black Death, which killed as much as 60 percent of the population of 14th Century Europe, or a similar outbreak need not happen."