  Background: National or local laws, norms or regulations (sometimes and in
some countries) require medical providers to report notifiable diseases to
public health authorities. Reporting, however, is almost always incomplete.
This is due to a variety of reasons, ranging from not recognizing the diseased
to failures in the technical or administrative steps leading to the final
official register in the disease notification system. The reported fraction
varies from 9% to 99% and is strongly associated with the disease being
reported.
  Methods: In this paper we propose a method to approximately estimate the full
prevalence (and any other variable or parameter related to transmission
intensity) of infectious diseases. The model assumes incomplete notification of
incidence and allows the estimation of the non-notified number of infections
and it is illustrated by the case of hepatitis C in Brazil. The method has the
advantage that it can be corrected iteratively by comparing its findings with
empirical results.
  Results: The application of the model for the case of hepatitis C in Brazil
resulted in a prevalence of notified cases that varied between 163,902 and
169,382 cases; a prevalence of non-notified cases that varied between 1,433,638
and 1,446,771; and a total prevalence of infections that varied between
1,597,540 and 1,616,153 cases.
  Conclusions: We conclude that that the model proposed can be useful for
estimation of the actual magnitude of endemic states of infectious diseases,
particularly for those where the number of notified cases is only the tip of
the iceberg. In addition, the method can be applied to other situations, such
as the well known underreported incidence of criminality (for example rape),
among others.
