The Science Book

196

A CONTAGIOUS

LIVING FLUID

MARTINUS BEIJERINCK (1851–1931)

T

hese days, the word “virus”

is all too familiar as a

medical term, and many

people understand the idea that

viruses are just about the smallest

of the harmful agents, or germs,

that cause infections in humans,

other animals, plants, and fungi.

Yet at the end of the 19th

century, the term virus was only

just making its way into science

and medicine. It was suggested

in 1898 by Dutch microbiologist

Martinus Beijerinck for a new

category of contagious disease-

causing agents. Beijerinck had

a special interest in plants and a

skilled talent for microscopy. He

experimented with tobacco plants

that were suffering from mosaic

disease, a discoloring mottled

effect on the leaves that was costly

IN CONTEXT

BRANCH

Biology

BEFORE

1870s and 80s Robert Koch

and others identify bacteria as

the cause of diseases such

as tuberculosis and cholera.

1886 German plant biologist

Adolf Mayer shows tobacco

mosaic disease can be

transferred between plants.

1892 Dmitri Ivanovsky

demonstrates that tobacco

plant sap passing through the

finest unglazed porcelain filters

still carries infection.

AFTER

1903 Ivanovsky reports

light-microscope “crystal

inclusions” in infected host

cells, but suspects they are

very small bacteria.

1935 US biochemist Wendell

Stanley studies the structure

of the tobacco mosaic virus,

and realizes that viruses are

large chemical molecules.

Tobacco mosaic disease shows features of an infection, but...

...filters that catch bacteria do not catch and remove

the contagion, so it cannot be bacteria.

So the causative agent must be different and

even smaller, deserving a new name—virus.

Also, unlike bacteria, the infectious agent grows only

in a living host, not in laboratory gels or broths.