| Viruses are particles composed of genetic material (DNA or RNA, but not both) surrounded by a protective protein coat. Outside a host cell, they are inert; inside, they enter a dynamic phase in which they replicate, pirating the host cell’s enzymes, nucleic and amino acids, and machinery to accomplish what they are not equipped to do alone. Viral replication is often carried out at the expense of the host: Diseases such as herpes, rabies, influenza, some cancers, poliomyelitis, and yellow fever are of viral origin. Of the estimated 1000 to 1500 types of viruses, approximately 250 cause disease in humans (over 100 of which cause the common cold), and 100 infect other animals. |
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