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Much of the progress in immunological and viral studies—indeed in medical research as a whole—has been due to genetics. Scientists now study the cells of the body and the organisms that may infect it at the molecular level. In the course of late 19th and early 20th century biochemical and genetic studies had revealed many of the basics of cell metabolism and the role of genes. By mid-century researchers understood the structure of genes and how they are arranged within the chromosomes that contain them. At the core of the chromosome is a long molecule called deoxyribonucleic acid, better known as DNA.

Then, in 1953, British biochemist Francis Crick (1916–2004) and American biologist James Watson (1928–…) decoded the structure of DNA. This was one of the great breakthroughs in science. Knowing the structure made it possible to determine the location of each gene and gradually to identify its particular purpose. By the beginning of the 21st century scientists had mapped the genetic structure of humans, the so-called human genome.

Beyond its importance to cellular studies, cracking the genetic code revolutionized medicine in a number of ways. The causes of many diseases can be traced to defective chromosomes or to specific genes on chromosomes. This in turn has made it possible to screen for susceptibility to such diseases, including cystic fibrosis, Huntington’s chorea, and some forms of breast cancer.

Genetic engineering has also made it possible to create new drugs derived from natural body chemicals. These include insulin, interferon, human growth hormone, and other hormones used to stimulate blood cell production. The greatest goal of genetic engineering is direct gene therapy. This involves inserting normal copies of abnormal genes into cells, usually by means of a virus. The hope is that gene therapy will offer cures for many diseases. So far, however, progress has been very limited.

Just as genetics has started to permit physicians to see the workings of the body at the most minute level, new technology has allowed them to see the body processes in action.