26.3Viruses, viroids 類病毒, and prions 普恩蛋白 are formidable pathogens in animals and plants

Viral Diseases in Animals
Viral infections cannot be treated by antibiotics
 * causing the release of hydrolytic enzymes from lysosomes
 * infected cells to produce toxins that lead to disease symptoms
 * envelope proteins that are toxic

Antiviral drugs can help to treat, though not cure, viral infections

Emerging Viruses新興病毒
Flu epidemics (流行性感冒)

pandemics (流感大流行)

“swine flu”(豬流感)

Viral Diseases in Plants
Most plant viruses have an RNA genome

Plant viruses spread disease in two major modes:

i. Horizontal transmission 平行轉染, entering through damaged cell walls

ii. Vertical transmission 垂直轉染, inheriting the virus from a parent

Viroids and Prions: The Simplest Infectious Agents
Viroids 類病毒are circular RNA molecules that infect plants and disrupt their growth

Prions 普恩蛋白 are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals

造成疾病(多為哺乳類動物的腦退化性疾病，因其會堆積在神經細胞):

(1) Scrapie 羊搔癢症-羊

(2) mad cow disease 狂牛症-牛

(3) Creutzfeldt-Jakob disease (CJD)庫賈氏病