(9 of 10)
Virus particles have been found in some human cancers, but this does not prove that they caused the disease. Some tumor viruses invade an animal, yet they disappear for months or years, and then belatedly cause cancer. The hows and whys of this latent period are unknown. One partial explanation may lie in the ability of new "provirus" particles to remain undetected in cells, doing no evident damage until they are stimulated by chemicals or X rays. The important thing is that these nucleic-acid molecules can be infective by themselves, with no assist from the protein that normally accompanies them in the whole virus. Dr. Frank L. Horsfall, director of Manhattan's Sloan-Kettering Institute, the world's biggest private cancer research organization, sums it up: "We can now speak of infectious molecules in animal as well as plant diseases—something that was inconceivable only a decade ago."
It may be that in some unpredictable cases, a molecule of viral nucleic acid, without its protein overcoat, so closely resembles a gene that it can slip into the cell's chromosomal lineup, displacing a normal gene, and make the cell reproduce abnormally. Most of the resulting abnormal cells would probably die, but a few might retain the power to run wild and perpetuate themselves as cancer.
Where these infectious molecules might come from and what might trigger them into activity at unpredictable times are still mysteries. Perhaps they are inherited, and lie dormant for decades. This would go far to explain why some cancers, though not hereditary in the ordinary sense, tend to run in families. Or they may come from virus infections of the mother during pregnancy: if they cross the placental barrier, they could lodge in the fetus, which has little or no antibody-forming mechanism to reject them.
Triggering mechanisms are still more obscure. Stanford University's Radiologist Henry Kaplan has shown that if he gives a dose of X rays to seemingly virus-free mice, they develop cancers containing virus particles. The late Dr. Francisco Duran-Reynals argued that chemicals and viruses combine to cause cancer. Now many laboratories are confirming his basic thesis: mice painted with a low dose of a known carcinogen (cancer-causing chemical) get no tumors, and neither do those exposed only to viruses; but if mice get both the virus and minute amounts of the chemical, many of them soon develop cancer.
Interferon. From the chemists' laboratories there has come no drug that will selectively attack viruses while sparing the cells in which they seek sanctuary. But nature suggests that there is a way. If it takes days or weeks for protective antibodies to develop, why does not the pullulating virus overwhelm all the victim's susceptible cells in the meantime? London's Dr. Alick Isaacs last year found a partial answer. Virus-infected cells produce a substance that Isaacs calls interferon, which spreads to neighboring, uninfected cells. With their interferon guard up, these cells are unusually resistant to viral invasion.