Fleas are among the most serious carriers of disease in all of recorded history. The Black Death or Bubonic Plague that wiped out a large percentage of the world's population during several waves during the Middle Ages was caused by fleas that picked up the bacteria from infected rats and transmitted it by biting humans. Plague is still sometimes spread by fleas, though it is no longer the same deadly disease, thanks to modern antibiotics. But as blood-feasting insects, fleas can also spread a variety of other diseases, including typhus. Of the 2,500 species of fleas found worldwide, there are roughly 300 that can be found in North America, and of these, four species are possible spreaders of disease: Cat fleas (Ctenocephalides felis) can transmit some types of typhus and cat-scratch disease (CSD). It commonly infests all types of warm-blooded domestic animals, not just cats. Rarely, it has been found to carry plague bacteria. This is the pest that most often infests pets in North America and the one that most often bites humans. Dog fleas (Ctenocephalides canis) are a prime carrier of the Dipylidium caninum tapeworm, which can infest dogs, cats, and even humans. Despite the name, it's not very common as a pest for dogs. Ground squirrel fleas (Oropsylla montana) are associated with ground squirrels and are known to be a factor in the spread of plague bacteria in North America. Oriental rat fleas (Xenopsylla cheopis) are common spreaders of plague and typhus. Some diseases are spread indirectly when a victim scratches bites, which then opens up the skin to bacteria found in flea feces or other sources. Other diseases, such as typhus, and parasites such as tapeworms, are transmitted directly by bites from fleas that have picked up pathogens from other sources, such as rodents or wild animals.