In the United States, there are about twenty-two million hearing-impaired people; of these, two million are profoundly deaf (unable to hear any thing) or severely deaf (unable to hear much). Hearing impairment results from three major factors that are not necessarily exclusive: environmental, hereditary and old age. Environmental causes include noise-induced, accidental, toxic, and viral. Noise-induced deafness is primarily a phenomenon of the modern industrial world, though stonemasons, may have been subject to hearing-loss in the ancient world. Permanent deafness resulting from toxicity is also a phenomenon of the modern world. Deafness from accident, such as a blow to the ear, must have resulted from time to time. Viruses, too, were very much part of the ancient world. Of the six main viruses that can cause deafness today—chickenpox, common cold viruses, influenza, measles, mumps, and poliomyelitis—there is evidence for five in ancient Greece. There is also evidence for the presence of bacterial meningitis, whose classic complication is hearing loss. In modern, developed countries, preventative medicine reduces the incidence and severity of these viruses, but in the ancient world, as in third-world countries today, these viruses must have taken their toll. There is no reason to rule out hereditary deafness in the ancient world, and there is some conjectural evidence for the results of in-breeding, although not specifically for deafness. In addition to inbreeding, other hereditary factors would have produced deafness. Some families simply have a genetic background that favors deafness.