For all of human history, people have observed the ill effects of diseases caused by poisons.
Whether it was a disease from a venomous snake, poisons extracted from frogs, or the accidental consumption of poisonous plants, humans learned early on to be knowledgeable about these causes of disease. However, at the same time, some diseases would occur where no explanation could be found. The person never ate, drank, or came in contact with something poisonous, but still experienced similar symptoms of lethargy, organ afflictions, and sometimes death. Early on, it was assumed that since poison-induced illnesses were well known, it may be possible that these other diseases were caused by something “poisonous” to our biology, though perhaps the culprit was unidentified or invisible to the eye. In the 19th century, a microscopic poison was hypothesized to exist, responsible for the diseases whose cause was elusive to our knowledge. This entity was given the name “virus” which in Latin means “toxin.”
Over the course of scientific progress, as inventors created microscopes and biological study become rigorous, people slowly realized that a microscopic, communicable toxin couldn’t be found. In many ways, the idea was slowly abandoned in favor of a new culprit, micro-organisms, which also became known around the same time.
However, research that sought to extract micro-organisms such as bacteria or fungi, and inflict disease states by way of transmission of these entities, failed to impress the scientific community with results. Organisms, by themselves, through normal routes (barring direct injection into the brain, abdomen, or lungs) couldn’t directly cause disease states.