What could cause someone to have a high lymphocyte blood count and sometimes run a low-grade fever at night?

This is a difficult question to answer. Any type of infection can caue the above symptoms. Anything from mononucleosis, malaria, lymphoma, strep, etc. the list is virtually endless. A GOOD history and physical exam by an experienced doctor will help him start to figure out which tests to order. It can be a lenghty process, since it is a process of elimination.