Epidemiology is the study of health and disease; specifically, the way that a population exists in a healthy state, or in a diseased state. Microbes refer to single-celled organisms. Thus, microbial epidemiology is largely concerned with the diseases that can be spread to humans by microbes.
I've included a link to the Yale School of Public Health, which includes a graduate division dedicated to the epidemiology of microbial diseases.
Some of the program requirements include courses on viruses and viral vectors, molecular biology, evolution, population modeling, statistics, and laboratory research work.
Keep in mind that "being" a microbial epidemiologist is dependent upon your context; the actual education required to be employed in the field may vary depending upon the employer, location and tasks involved. Generally speaking, though, a career like this will require at least a Masters level of education in order to fully understand breakthroughs in the science and technology that underly the field, and a Ph.D. level in order to perform research and experiments.