In 1546 Girolamo Fracastoro wrote a classic description of measles when he theorised that the disease was caused by “seeds” (seminaria) that were spread from person to person. Despite these early considerations, from ancient times until the mid 19th century the prevailing theory of disease was miasmatic theory. Miasma was considered to be a poisonous vapour or mist. The belief held that most, if not all disease was caused by inhaling air, which was infected through exposure to corrupting matter like rotting vegetation. Many believed miasma was magical, and was able to change the properties of the air and atmosphere completely. In the mid of the 19th century the miasma theory was replaced by the germ theory of diseases and this was consolidated when the first images of viruses were obtained through the invention of electron microscopy in 1931.
After the existence of viruses was determined, the attention shifted to the origins of their existence, with some even believing in the unconventional Pseudo-panspermia hypothesis, which argues that the pre-biotic organic building-blocks of life originated in space and were distributed to planetary surfaces where life then emerged. Proponents of this hypothesis, Hoyle and Wickramasinghe, have speculated that several outbreaks of illnesses on Earth are of extraterrestrial origins.
Viruses have always had a reputation for being harbingers of death, however the sheer abundance of viruses that have recently been uncovered and their overwhelming presence in many ecosystems, has led modern virologists to reconsider their role in the biosphere. Viruses are the main agents responsible for maintaining the ecological balance of different species of marine blue-green algae and therefore adequate oxygen production for life on Earth.