KEMRI is an employer just like any other in Kenya and therefore there are no specific or special subjects to study in order to work there. What would be important is the type of job that you desire to do. While KEMRI is know for research, it also have multiple other positions like in human resource management, accounts, computers/information technology etc. So what the employer will look at is if you have the qualification for the position you are applying for.
If you want a position in research the entry position is called an assistant research officer and the basic qualification fro this is a university degree in a relevant field.