Smallpox is the classical example of this, but most large predators (wolves, bears, etc) have been driven to extinction in locations where humans are found in large numbers.

You could argue that the Tasmanian tiger was driven to extinction for commercial reasons as it was hunted to protect sheep flocks [1].

1. https://en.wikipedia.org/wiki/Thylacine

Also, the Guinea worm eradication program is getting quite close to succeeding. (http://www.healthmap.org/site/diseasedaily/article/guinea-wo...)

To save, maybe not. But we sure did it for other reasons -- like say to hurt others, or for economic reasons. One example is:

https://en.wikipedia.org/wiki/Bison_hunting

We went from 30 million on North American continent around 16th century, to a few hundred around 19th century.

Smallpox is the best example. Today, it exists only in closely-guarded labs. In prehistory, we also wiped out all the megafauna outside Africa --- partly because they were delicious, I'm sure, but also partly because they found us delicious.

Just about every large predatory animal ever.

The polio virus comes to mind.

Neanderthals. Perhaps it can be considered first world war.

Most big cats in Europe. Possibly Aurochs.