Negative for mono. what other sicknesses have similar symptoms?

Mono test vs syndrom: Mono is a lay term for an illness most often (80%) caused by the EB virus. The office test may be falsely negative early in the illness. There are other viruses and infections (20%) that can cause the monocytes to show up in high numbers in the blood, with a vague flu like illness that is similar to mono. Defining the cause of a non EB virus mono is difficult and would require specialty lab.