-#' @param mu Mean of the serial distribution. This must be a positive number or
-#' `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 `mu` should be set to `1`. If
-#' case counts are daily and the serial distribution has a mean of seven days,
-#' then `mu` should be set to `7`.
+#' @param mu Mean of the serial distribution (defaults to `NA`). This must be a
+#' positive number or `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 `mu` should be set
+#' to `1`. If case counts are daily and the serial distribution has a mean of
+#' seven days, then `mu` should be set to `7`.