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 DNAand later RNA. Heinz Fraenkel-Conrat and Robley Williams helped develop the understanding that viruses had genetic structures like other living creatures. Viruses now became classified as other living beings in the hierarchy of life. 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. Complex DNA makeup and rapid rates of mutation has meant that new viruses could form that are unknown to scientists, including viruses that can go between humans and other life.<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>