We are also responsible for a bunch of misplaced plant and animal species. Some are intentional, like bullfrogs in California, trout in western streams, and the marine toad in Australia. Some are not, riding on bundles of fruit, on ships, or escaping captivity. Just to please sport fishermen, trout have been introduced into many previously fishless mountain streams where frogs were the top predator . Frog eggs and tadpoles fit nicely into the trout's natural diet, not to mention its mouth.
Fleas are parasites.
Measles and mumps are diseases. I guess once in a while a new disease or parasite can come in and do heavy damage 'cause the plants and animals aren't ready for it. A freshwater bacterium is causing the legs of some western frogs to get red and puffy. This condition means the frogs immune system is kaput. A microscopic critter that was introduced by stocked fish, Saprolegnia ferax, is killing off amphibian eggs.