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what was responsible for this neat passage of genetic information, from the tiniest virus to the largest elephant. That year, Oswald Avery and his colleagues at the Rockefeller Institute in New York showed that if they destroyed a specific molecule found inside all living cells, the organisms could no longer pass on their genes.
    The molecule was called deoxyribonucleic acid, or DNA.
    In 1953 researcher Rosalind Franklin, working at King’s College in London, made the first X-ray photographs of DNA, showing that the molecules had a unique helical structure composed of various combinations of the same five key chemicals.
    Later that year, America’s James Watson and Britain’s Francis Crick, working at Cambridge University, figured it all out. One of the chemicals—a sort of carbon chain linked by powerful phosphate chemical bonds—created parallel curved structures similar to the poles of a long, winding ladder. Forming the rungs of the ladder were four other chemicals, called nucleotides. The order of those nucleotide rungs along the carbon/phosphate poles represented a code which, when deciphered properly, revealed the genetic secrets of life.
    DNA, then, was the universal code used by one meningococcal bacterium as the basis for making another meningococcal bacterium. It was the material wrapped up inside the chromosomes of higher organisms. Sections of DNA equaled genes; genes created traits. When the chromosomes of one parent combined with those of another parent, the DNA was the key, and which traits appeared in the children (blue versus brown eyes) was a function of the dominant or recessive genes encoded in the parents’ DNA. 11
    While government officials were bragging that everything from malaria to influenza would soon disappear from the planet, scientists were just beginning to use their newfound knowledge to study disease-causing viruses, bacteria, and parasites. Scientists like Johnson were of the first generation of public health researchers to know the significance of DNA. Understanding how DNA played a direct role in the emergence of disease would take still another generation.
    Starting at nature’s most basic level, scientists at Cold Spring Harbor Laboratory on Long Island, New York showed in 1952 that viruses were essentially capsules jam-packed with DNA. Much later, researchers discovered that some other viruses, such as polio, were filled not with DNA
but with its sister compound, RNA (ribonucleic acid), which also carries the genetic code hidden in sequences of nucleotides.
    When Karl Johnson was virus hunting in Bolivia, scientists had a limited understanding of the vast variety of viruses in the world, the ways these tiniest of organisms mutate and evolve, or how the microbes interact with the human immune system. The state of the art in 1963 was best summarized in Frank Fenner’s animal virus textbook, the bible for budding microbiologists of the day:
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    Suppose that we have isolated a new virus and have managed to produce a suspension of purified particles. How can we classify the virus, and how do we find out about its chemical composition? A lead may be provided by its past history —the species of animal from which it was isolated and whether or not it was related to a disease. This information, in conjunction with that obtained by electron microscope examination of … particles, might be enough for us to make a preliminary identification. 12
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    Scientists could “see” viruses with the aid of microscopes powerful enough to magnify up to visual level objects that were nearly a million times smaller than a dime. With that power of magnification they could detect clear differences in the appearance of various species of viruses, from the chaotic-looking mumps virus that visually resembles a bowl full of spaghetti to the absolutely symmetrical polio virus that looked as if it were a Buckminster Fuller-designed sphere composed of alternating