Of the many different methods used to calculate R 0 , only the survival function reliably calculates the average number of secondary infections; however, this method is too cumbersome to use in most practical settings. The nextgeneration method is probably the most popular, yet it suffers from uniqueness problems and does not cope well with more than one disease state. Since R 0 is rarely measured in the field, it instead relies upon after-the-fact calculations to determine the strength of disease spread [2] . This limits its usefulness even further.