Invasive species are nothing new to the world. However, it has accelerated with increases in world trade and travel. Sometimes species are introduced by accident and other times by bad decisions made by humans. Some of them almost sound like plots for science fiction movies. The spread of the Black Death in medieval Europe and the Zika virus in the past year are two examples that span the centuries.
There are other examples that don't impact human health as dramatically, but can have severe consequences to native species and the economy of large regions. Two invasive species being dealt with now in the U.S. are the Asian carp and the emerald ash borer.
This is an example of a totally avoidable problem. The Asian carp were introduced into the U.S. for cleaning up the water in commercial catfish farms. Apparently no thought was given to the possible consequences or we were arrogant enough to think we could contain the carp. What could go wrong? They did escape and spread north soon to reach the Great Lakes in the northern midwest region of the U.S. The large and prolific carp compete with native species of fish to damage both recreational and commercial fishing. They are also a danger to boaters. A 40 pound (18 kg) carp jumping 10 ft (3 m) out of the water can be a surprising and painful experience.