Changes

Variolation continued to be practiced in Europe and Asia in the 18th and 19th centuries, which undoubtedly began to limit viral deaths, particularly from smallpox. However, viruses, particularly smallpox, led to large-scale outbreaks and this continued to limit the average age of populations to around 40 years even by the early 19th century. Overcrowded conditions and poor sanitation made things worse particularly for the poor. Nevertheless, there was a realization that vaccination could help prevent the spread of smallpox. This lead to the first compulsory vaccination act, called the Vaccination Act of 1853, which led to free vaccinations for smallpox and made it enforced for anyone over the age of three months. The law was not well enforced and outbreaks continued to occur. One major breakthrough in virus treatment came from Louis Pasteur. He realized that taking spinal cords from dogs that died from rabies could be used to immunize people from rabies. He began crushing these spinal cords of dead dogs and injecting them into health dogs, which went on to survive. However, viruses were still not understood. A key development happened in the 1880s when Charles Chamberland developed a filter small enough to allow bacteria through but not other microbes. This allowed, in 1892, Dmitri Ivanovsky to isolate the agent causing the plant sickness, although he thought it was a toxin and not a virus. In 1898, Martinus Beijerinck became convinced that some other active agent, what he would later call a virus, was likely causing sicknesses in plants.<ref>For more on variolation and early laws to encourage vaccination, see: Williams, Gareth. 2010. <i>Angel of Death: The Story of Smallpox</i>. Basingstoke: Palgrave Macmillan.</ref>