Changes

It was not until 1931, when Ernst Ruska and Max Knoll developed an early form of electron microscope (Figure 2), that it now became possible to observe the agents that caused viral spread. Viruses could now be observed and their structure described. 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.<ref>For more on viruses discovered and the history behind learning more about them in the 20th century, see: Emmer, Rick. 2006. <i>Virus Hunter. Weird Careers in Science.</i> Philadelphia: Chelsea House Publishers., pg. 12</ref>