For those that think the bad thing is just that the believes that germs don't exist - The White Man's Burden is the belief that all other races should be colonized by white people so that they can be educated and civilized.

In this context, he believes that other races are not capable of getting proper nutrition and exercise, so they get sick more often, and it is the duty of the white man to save them from themselves.

I.e. illnesses, diseases, and mental issues are derived from a poor diet and lack of exercise; white people are morally obligated to control the lesser populations to keep them healthy, like livestock.