The term "virus" is derived from the Latin word forposition, or slime. It was originally a

The term "virus" is derived from the Latin word for

position, or slime. It was originally applied to the noxious

stench emanating from swamps that was thought to cause a

variety of disease in the centuries before microbes were S1.______

discovered and specifically linked to illness. But it was S2.______

until almost the end of the nineteenth century that a true

virus was proven to be the reason of a disease. S3.______

The nature of viruses made them impossible to detect

for many years, even after bacteria had been discovered and

studied. Not only are viruses too small to be seen with a

light microscope, they also cannot be detected through their S4.______

biological activity, except as it occurs in conjunction with

other organisms. In fact, viruses show no traces of biological

activity by themselves. Unlike bacteria, they are not living

agents in the strictest way. Viruses are very simple pieces S5.______

of organic material composing only of nucleic acid, either S6.______

DNA or RNA, enclosed with a coat of protein made up of S7.______

simple structural units. (Some viruses also contain

carbohydrates and lipid.) They are parasites require S8.______

human, animal, or plant cells to live. The virus replicates

by attaching to a cell and inject its nucleic acid; once S9.______

inside the cell, the DNA or RNA that contains the virus'

genetic information takes on the cell's biological machinery, S10.______

and the cell begins to manufacture virtual proteins rather

than its own.

【S1】