We sometimes hear about nasty diseases for which we don't have cures, like Lyme disease and the Hantavirus. There are lots of ways to tackle these problems, but one lies a bit outside the usual medical arena.
Some diseases are spread through contact with animal carriers. A tick with Lyme disease bites a mouse and the mouse gets infected. The Lyme disease causing bacteria gets into new ticks when they bite an infected animal. Eventually, an infected tick bites a human and the human gets Lyme disease.
We haven't yet found a way to get rid of Lyme disease once a person is infected. Getting rid of infected ticks or mice is hard, as is keeping them away from people. There might be another way.
Studies are showing that loses in biodiversity correlate with increased spread of certain diseases. When there are lots of different kinds of animals, the ones that are good carriers are less prominent. In the Lyme disease case, ticks have more chance of biting hosts that can't carry the disease if there are plenty of other kinds of animals around besides mice.
The Hantavirus is another example. This virus is spread between small mammal species, but doesn't affect all related species. When there are a variety of animals around, there seems to be a lower incidence of the disease. When there are lots of other kinds of animals around, infected animals just may not come in contact with as many animals of their species and so can't spread the disease as fast.
In this case, more rodents, at least more kinds of rodent, is a good thing for our health.