Until the 19th century most illnesses were attributed to divine displeasure or to low-quality air - malaria was named from the Italian for 'bad air'. But diseases such as leprosy were known to be 'catching' from Old Testament times, when all lepers were declared 'unclean' and forced into isolation.
In the 1st century BC the Roman encyclopedist Marcus Terentius Varro speculated that disease might be caused by minute particles entering the body, and in the 6th century AD the Hindu doctor Susruta suggested that malaria might be spread by mosquitoes. But the realisation that people or things could transmit 'plagues' was not accepted until the Middle Ages. From about 1380, after the plague or Black Death had exterminated nearly a quarter of Europe's population, ships carrying infection were refused entry to Venice. At Ragusa on the Adriatic, immigrants and traders had to remain outside the city for 40 days to prove they were not infected. This was known as quarantinza, from the Italian quaranta, 'forty', hence the word 'quarantine'.
Epidemics, suggested the Italian physician Giraolamo Fracastoro in 1546, were caused by 'seeds' wafted through the air or carried in water. Much later the microscope would help to confirm the principle, but as early as 1683 the instrument's Dutch inventor Anton van Leeuwenhoek probably viewed bacteria from his teeth by means of its powerful lens. In 1850 Casimir Davaine showed that anthrax could pass in the blood from infected to uninfected sheep and cattle, and detected rod-shaped anthrax bacilli in newly infected animals.
Even the most powerful microscopes could not detect viruses, the minute agents responsible for diseases such as polio. In 1892 the Russian bacteriologist Dmitry Ivanovski discovered that the agent of tobacco mosaic disease, which affects tobacco leaves, was not filtered out by a fine mesh that trapped bacteria. In the late 1930s, when the electron microscope was invented, viruses were finally observed directly.