Viruses

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Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes

Some viruses cause infected cells to produce toxins that lead to disease symptoms

Others have envelope proteins that are toxic

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Vaccines are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen

Vaccines can prevent certain viral illnesses

Viral infections cannot be treated by antibiotics

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

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Emerging Viruses

Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists

Severe acute respiratory syndrome (SARS) recently appeared in China

Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

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Flu epidemics are caused by new strains of influenza virus to which people have little immunity

Viral diseases in a small isolated population can emerge and become global

New viral diseases can emerge when viruses spread from animals to humans

Viral strains that jump species can exchange genetic information with other viruses to which humans have no immunity

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These strains can cause pandemics, global epidemics

The “avian flu” is a virus that recently appeared in humans and originated in wild birds

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Fig. 19-9

(a) The 1918 flu pandemic

(b) Influenza A

H5N1 virus

(c) Vaccinating ducks

0.5 µm

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Fig. 19-9a

(a) The 1918 flu pandemic

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Fig. 19-9b

(b) Influenza A H5N1

virus

0.5 µm

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Fig. 19-9c

(c) Vaccinating ducks

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Viral Diseases in Plants

More than 2,000 types of viral diseases of plants are known and cause spots on leaves and fruits, stunted growth, and damaged flowers or roots

Most plant viruses have an RNA genome

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Fig. 19-10

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