Nausea is a tier 1 symptom found in all of the standard plague types, the Simian Flu DLC, and the Smallpox scenario. It makes the infected's stomach lining irritated which leads to discomfort. When kissing, there is a chance that one might get infected.
During the Festival of Love, if one has evolved Nausea, infectivity rates will skyrocket in said country.
Nausea is the symptom the least likely to be noticed, meaning you can "evolve" this symptom without alerting authorities just like the Coughingsymptom.