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. 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. It was not until 1931, when Ernst Ruska and Max Knoll developed an early form of electron microscope, that it now became possible to observe the agents that caused viral spread. By the 1950s, as viruses were increasingly observed, it now also became possible to study their structure through the understanding of DNA. Heinz Fraenkel-Conrat and Robley Williams helped develop the understanding that viruses had genetic structures like other living creatures. Eradication campaigns that began in the post-World War II era have now successfully mostly eradicated smallpox. Today, over 2000 viruses affecting animal, plant, and bacterial life are known, but potentially millions of varieties exist.