Thanks to the trade and imperial ventures of humans, these disease-causing organisms could cross regional boundaries and spread infection.
Humans are really strange. They love giving awkward names to other living beings. For example, I am called a 'virus'. In Latin, it means poison or poisonous liquid. Well, humans do seem to enjoy giving names carrying negative connotations. My other name, 'pathogen', is not encouraging either. So, what is a pathogen? If you Google it, you will get the following explanation: 'Pathos' is the Greek word for disease and genes means 'born of'.