Bill Bryson

in isolation viruses are inert and harmless, but introduce them into a suitable host and thy burst into life.  About 5000 types of virus are known and …. they afflict us with many hundreds of diseases ranging from flu and common cold to smallpox, rabies, yellow fever, ebola, polio and AIDS.

Viruses prosper by hijacking the genetic material of a living cell and using it to produce more virus.  They reproduce in a fanatical manner, then burst out in search of more cells to invade.  Not being living organisms themselves, they can afford to be very simple.  Many, including HIV, have ten genes or fewer, whereas even the simplest bacteria require several thousand.  They are also very tiny, much too small to be seen with a conventional microscope.  It wasn't until 1943 and the invention of the electron microscope that science got its first look at them.  But they can do immense damage.  Smallpox in the 20thC alone killed an estimated 300 million people.

Bill Bryson

Ebola, Lassa and Marburg fevers  have all tended to flare up and die down again …...... and it is apparent that the virus which causes AIDS has been among us much longer than anyone originally suspected.  Lassa fever, detected in 1969 in West Africa, is extremely virulent and little understood.

Viruses also have an unnerving capacity to burst upon the world in some new and startling form and then to vanish again as quickly as they came.

For example:-

• In 1916, people in Europe and America came down with a disease called encephalitis lethargica.  It killed 5 million people and then went away

• The Great Swine Flue epidemic/Great Spanish Flue epidemic killed millions 'No one knows the global toll, but it was between 20 to 50 million people.  Some estimates  have put the global total as high as 100 million.  The flu epidemic erupted suddenly, in places separated by oceans, mountain ranges and other earthly impediments.  A virus can survive for no more than a few hours outside a host body, yet it appeared in Madrid, Bombay and Philadelphia in the same week.  The explanation often given is that it was spread by human carriers who suffered no symptoms, but this does not explain the simultaneous nature of the epidemic'. The very young and older people were relatively unaffected, the only ones affected were those of 'breeding age' – in their twenties and thirties

• A disagreeable Russian virus known as HYNI caused an outbreak of disease in 1933, 1950 and 1970, but with no return in the meantime

L'Evolution du Vivant 1973 – Grasse

…........there exists a molecular mechanism which, in certain circumstances, supplies information from outside the organism and inserts this information into the organism's genetic code.