I got curious about the differences so looked them up-
http://www.nlm.nih.gov/medlineplus/mplusdictionary.html

dis·ease
Function: noun
: an impairment of the normal state of the living animal or plant body or one of its parts that interrupts or modifies the performance of the vital functions, is typically manifested by distinguishing signs and symptoms, and is a response to environmental factors (as malnutrition, industrial hazards, or climate), to specific infective agents (as worms, bacteria, or viruses), to inherent defects of the organism (as genetic anomalies), or to combinations of these factors : SICKNESS : ILLNESS -- called also morbus -- compare HEALTH 1

syn·drome
Function: noun
: a group of signs and symptoms that occur together and characterize a particular abnormality

con·di·tion
Function: noun
: a usually defective state of health <a serious heart condition> b : a state of physical fitness <exercising to get into condition>