A virus is a simple, tiny genetic organism made from protein, that contains DNA or RNA.  The structure of a virus consists of a nucleic acid surrounded by a resilient protein coat.  It can exist either inside or outside a host cell, but needs to be "linked" to a host in order to reproduce.  Viruses are obligate intracellular parasites, which means that they are not able to reproduce outside their living host.  They operate by invading cells and hi-jacking them for their own benefit.  It needs the "machinery" of a living host cell to carry out its reproduction.  Without a cell, each is inert and does nothing.  Viruses can, however, survive in the inert state for prolonged periods - perhaps thousands if not millions of years.  They can then "activate" when the right conditions come along.  Viruses multiply by using the host cell to replicate the viral particle.  This can and often does kill the host cell and the viruses it has produced move on to invade new host cells.  Viruses reproduce rapidly and evolve at alarming rates.  Most viral infections can be detected by serology, which is the study of what kills what (antigen-antibody reactions) in the laboratory.Viruses can be a variety of shapes - spherical, cubic clumps and cylindrical clumps, for instance.
Orchid Virus.  It is the bullet shaped structure in a spoke wheel configuration
© 1999 The Centre for Microscopy and Microanalysis

Because viruses do not exhibit all the seven characteristics of life
when unattached to a host cell, it is debatable whether or not
they are truly living entities. However, they may have a severe impact on the health of the cells they get into, by degrading the cells' ability to thrive.  This is known as infection.  Viruses are much smaller than bacteria, and an electron microscope has to be used to view them.   Viruses range from 10 to 800 nanometers (0.000000001  to 0.00000008 mm) in length, while normal bacteria are around ten times that size.  Viruses tend to be specialized entities that are species specific.  However, they frequently jump the species barrier with devastating effect.  Influenza viruses, for instance, derive from bird and pig viruses.  Viruses can attack all three domains of life.

Influenza
Influenza virus is probably the best known and most common virus among the human population.  It mutates at an alarming rate and new strains appear on an annual basis.   The occurrence of influenza peaks with the northern winter.  This is because the population in the northern hemisphere is larger than that in the south and human habits - being indoors in warm damp conditions with lots of other people and then being outdoors in the cold - promote the spread of the disease.  Once a person has had a particular strain they are immune to that strain subsequently.  Sometimes this immunity extends to being effective or partially effective against future strains, but usually new strains of influenza are able to attack anyone.   The disease has many symptoms.  Some or all of these may occur: high temperature,  fever, nausea, runny nose, sore throat, cough, congestion of the lungs and airways.  In old people in particular, it can lead to serious respiratory diseases and secondary infections such as pneumonia, and this can kill.

Influenza viruses originate largely in the bird population.  In the wild, such viruses can spread quickly when migratory birds gather for mating or flying to summer or winter habitats.  But this does not explain how the virus enters the human population.  The answer is that generally, it "leaps the species gap" in domestic birds such as chickens.  In addition, some strains of influenza are known to have originated as hybrid chicken-pig viruses.  Very often new influenza strains seem to originate in the Far East, where domestic fowl, pig and human populations are numerous and often live in close proximity to one another.

Pandemic
The world's worst  influenza epidemic occurred in 1919 just after the First World War.  It killed nearly 22 million people, which was more than the war did!  It was unusual in that the death toll was greatest among younger people, rather than the elderly, as normally occurs.   It has been speculated that immunity picked up by the older population in the 1880's or 1890's had been partially effective.

Viruses can infect most types of living cells, even bacteria.  Such a virus is known as a bacteriophage and can be used to control bacterial infections.  These viruses have a particularly unusual shape, not unlike a rocket or a stretched Apollo Lunar landing vehicle!  Viruses can also enter a dormant state by crystallizing to be stored like minerals, where they can remain unchanged for prolonged periods.  When moistened and provided with a favorable environment, they can revive for renewed activity.  They are therefore highly resilient and difficult to destroy.

T4 Bacteriophages.  The virus is the rocket-shaped organism that is linked to the cell above.  © 1999 The Centre for Microscopy and Microanalysis