Antibiotic-resistant bacteria. We hear about it but I feel that for the most part, we tend to think that it's a problem that somebody else will tend to and figure out.
Totally agreeing. So called "super-germs" are on the rise and the most immediate threat to us. There are more and more infections which can be treated only with some hard antibiotics but standard antibiotics do not work anymore. Bacteria getting more aggressive and resistant to new antibiotics quicker than before. Certainly, this development is only partially due to wrong medical application.
It is mostly because of the antibiotics used in agriculture, especially in chicken, turkey and pork "mass-cultures". We consume antibiotics with the meat we eat an let allowing resistant bacteria to thrive on the one hand. On the other hand, the animals themselves produce multi-resistant bacteria which are easily transferred to humans if we do not handle the meat correctly.
But as many other mentioned, this also comes with our greed and exploitation of the resources of this planet. We wouldn't have that problem (to this extent), if the food production was free range and organic, rather than mass production, where animals need antibiotics so they don't get sick.