epidemic; outbreak of disease
Refers to the spread of an illness within a population. Used in medical, news, and everyday contexts.
An influenza epidemic is a concern this winter.
The public health center urged caution to prevent an outbreak of disease.
流行 alone can mean 'fashion' or 'prevalence' of anything; 病気の流行 specifically means an epidemic of disease.
感染症 is 'infectious disease'; 病気の流行 is the outbreak or epidemic itself.
Compound of 病気 (disease) + の (possessive particle) + 流行 (spread, prevalence).