In 1898, Friedrich Loeffler and Paul Frosch found evidence that the cause of foot-and-mouth disease in livestock was an infectious particle smaller than any bacteria. This was the first clue to the nature of viruses, genetic entities that lie somewhere in the grey area between living and non-living states.
Viruses are small living organisms that are responsible for more suffering and disability than almost anything I can think of. They cause most of the childhood diseases such as measles, mumps, roseola and chickenpox. Contrary to common belief, you only get a specific virus once and then you have lifelong immunity to that specific virus. There are exceptions, but they are so rare as to not be clinically significant.
