Elizabeth Kolbert's book The Sixth Extinction tells the story of the Panamanian Golden Frog, a species that was nearly wiped out by an unfamiliar fungus. Kolbert shares a theory of how this fungus was carried to the frog’s population:
One theory — it has been very difficult to pin down — but it’s that this fungus was moved around the world. Another really interesting story on frogs [is that they] were used in the ’50s and’60s for pregnancy tests. Something called the African Clawed Frog, if you inject it with the urine of a woman who is pregnant, it will lay eggs very quickly. And obstetricians used to keep whole tanks of these frogs in their offices. And the African Clawed Frog turns out to be a frog that carries this fungus but doesn’t seem to be killed by it. So one theory is that as these frogs were [exported] around the world, they carried this fungus with them … so we brought the frogs and the frogs brought the fungus.
illustration by David Hughes, The New Yorker