Hindawi Publishing Corporation Computational and Mathematical Methods in Medicine Volume 2011, Article ID 527610, 17 pages doi:10.1155/2011/527610 Review Article The Failure of R0 Jing Li,1 Daniel Blakeley,2 and Robert J. Smith?3 1 Department of Mathematics, Pennsylvania State University, University Park, State College, PA 16802, USA 2 School of Mathematics and Statistics, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK 3 Department of Mathematics and Faculty of Medicine, The University of Ottawa, 585 King Edward Avenue, Ottawa ON, Canada K1N 6N5 Correspondence should be addressed to Robert J. Smith?,
[email protected] Received 30 January 2011; Revised 18 May 2011; Accepted 18 May 2011 Academic Editor: Haitao Chu Copyright © 2011 Jing Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The basic reproductive ratio, R0, is one of the fundamental concepts in mathematical biology. It is a threshold parameter, intended to quantify the spread of disease by estimating the average number of secondary infections in a wholly susceptible population, giving an indication of the invasion strength of an epidemic: if R0 < 1, the disease dies out, whereas if R0 > 1, the disease persists. R0 has been widely used as a measure of disease strength to estimate the effectiveness of control measures and to form the backbone of disease-management policy. However, in almost every aspect that matters, R0 is flawed. Diseases can persist with R0 < 1, while diseases with R0 > 1 can die out.