Diseases caused by bacteria and protozoa have been significantly reduced in the last 50 years or so. This is not true only for the developed countries, but also developing countries.
Better hygiene, development of new medical procedures have all contributed to reduction of some diseases.
However, the same can not be said for diseases caused by viruses. Big improvement were made in this area too, such as prevention of viral diseases by immunization, but still more has to be accomplished.
A virologist can greatly help in the advancement of the field of medicine and in the process help humankind.
How to become a virologist is a relatively simple and more or less outlined process, once you commit yourself to the field and in a larger sense have a drive to help humankind.