-\item{mu}{Mean of the serial distribution. This needs to match case counts
-in time units. For example, if case counts are weekly and the
-serial distribution has a mean of seven days, then \code{mu} should
-be set to one. If case counts are daily and the serial distribution
-has a mean of seven days, then \code{mu} should be set to seven.}
+\item{mu}{Mean of the serial distribution. This must be a positive number.
+The value should match the case counts in time units. For example, if case
+counts are weekly and the serial distribution has a mean of seven days,
+then \code{mu} should be set to \code{1}. If case counts are daily and the serial
+distribution has a mean of seven days, then \code{mu} should be set to \code{7}.}