12

CHAPTER

185

Viruses, Viroids,

and Prions

In this chapter, you will learn about viruses, how they work, and the diseases
they cause. You will also learn about viroids and prions. A viroid is a small
“naked” infectious RNA molecule that has similar properties of a virus. Prions
are particles of proteins and cause infections.

Viruses

In 1889, Dutch plant microbiologist Martinus Beijerinck described the concept
of viruses through his studies of Tobacco Mosaic Disease. Nobel prize–winner
Sir Peter Medawar described how microbiologists feel about viruses when he
said, “A virus is a piece of bad news wrapped in a protein.”
Viruses are strands of nucleic acids that are encased within a protein coat,
making them difficult to destroy. A microorganism needs both DNA and RNA in
order to reproduce. A virus cannot express genes without a host, because a virus
has either DNA or RNA but not both.

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