Emily

病菌：
virus、germ、bacteria

差別：
germ 一般指可引起疾病的細菌
A small simple structure (as a fertilized egg) from which new tissue can develop into a complete organism.

bacteria 兼指有害、無害甚至有益的細菌
Single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission; important as pathogens and for biochemical properties; taxonomy is difficult; often considered plants.

Virus 則是比細菌還要小的病毒，
可引起感冒(flu)、麻疹(measles)、小兒痳痺症(polio)、天花(smallpox)、狂犬病(rabies)、愛滋病(AIDS)等傳染病。例如：AIDS is caused by a virus that attacks the immune system（受滋病由一種攻擊免疫系統的病毒引起）。嚴格而言，virus本身不是流行病，但俗語卻常用這個字來說流行病
Ultramicroscopic infectious agent that replicates itself only within cells of living hosts; many are pathogenic.
資料來源： 蘋果日報