Viral fitness: definitions, measurement, and current insights

► Most viral fitness work examines replicative fitness within hosts or in cultured cells. ► There is increasing study of transmission fitness and epidemiologic fitness. ► A wide diversity of viruses are studied for fitness in vitro, in vivo, and ex vivo. ► A recent advance for vertebrate viruses is more assessment of fitness in vivo. ► Major topics include fitness in drug resistance, immune escape, and viral emergence.