What percent of cases of people with hearing impairment is caused by an undiagnosed chronic viral infection?

We see a good general rule of thumb being that 90% of hearing loss is due to chronic viral attack on the inner ear. As you know, susceptibility to infection has many adjuncts as predisposing factors and reasons for vulnerability in the first place. So there are often multiple adverse influences converging to get the problem started. Then, whether it tapers off or worsens will depend on many factors, including the general well-being, lifestyle, and average stress level of the victim. Thus, there are many confounding elements here in why there is such a variety of presentations and prognoses. But the most damaging factor is the potential of a chronic viral infection to cause tissue destruction as a consequence of a response by the immune system to contain the invading virus, and producing inflammatory changes that damage delicate cells of this sensory organ.