病毒の感染
The disease-germs have spread.
病毒の伝播
病毒の媒介物
病毒の潜伏期
the incubation period―the latent period
The fly serves as a medium for―serves as a vehicle for―the dissemination of disease-germs―The fly carries germs―conveys germs―The fly is an agency for the spread of germs.
Flies carry disease-germs on their legs.
Disease-germs are disseminated by the agency of air or water.