-\item{mu}{Mean of the serial distribution. This must be a positive number or
-\code{NA}. If a number is specified, 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}.}
+\item{mu}{Mean of the serial distribution (defaults to \code{NA}). This must be a
+positive number or \code{NA}. If a number is specified, 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}.}