Calculates Basic Herd Immunity Threshold Based on
the Basic Reproduction Number

## Arguments

- R0
a positive double, the basic reproduction number

## Examples

```
# Calculate the Herd Immunity Threshold for R0 Between 1 and 9
o <- vapply(seq(1,9, .1), hit, FUN.VALUE = numeric(1))
plot(seq(1,9, .1), o, main = "HIT", type = "l",
ylab = "Prop with Immunity", xlab = expression(R[0]))
```