Just one rat can be enough to cause a minor panic under the right circumstances. But swarms of rats can be devastating to crops and, according to isolated reports, dangerous to people.
When bamboo flowers are in bloom in northeastern India, rats take advantange of the increase in the food supply by breeding in larger numbers than unusual, causing their numbers to swell, according to BBC News. About every 50 years, this event isn't limited strictly to bamboo seeds, but often spills over into rice crops, threatening the livlihoods of poor farmers.
Rats typically aren't aggressive toward humans, usually fleeing at the sight of danger. But in Moscow, the Russian newspaper Pravda reported in 2009 that swarms of rats attacked pedestrians in Preobrazhenskaya Square. In South Africa, there were two gruesome reports in 2011 of giant rats, roughly the size of cats, killing infants.