Q: How to formulate a clear statement of research problem?
A statement of a problem is a brief, clear, and precise explanation of an issue that the researcher wants to study. It includes a vision, issue statement, and proposed method to solve the problem. The 5 'W's - Who, What, Where, When and Why - can be used to define a problem statement. I understand that you want to study the factor facilitating measles outbreak in a particular district. Since I am not sure what aspects you are exactly looking at or want to study, it would not be possible for me to suggest a statement of the problem.
However, we have published quite a few articles that would help you to understand how to write the statement of a problem.
Please go through these tutorials carefully and try to come up with a problem statement for your research. You can also discuss your ideas with your professor or seniors who might help you to come up with a framework and you can fill in the details depending upon what you are explicitly studying: