Update Xmod-SIS.Rmd

wording and formatting changes.

vignettes/Xmod-SIS.Rmd

@@ -15,39 +15,41 @@ library(deSolve)
 suppressMessages(require(knitr))
 ```
 
-# {.tabset}
-
-## $\odot$  
 
 The SIS model, as it is now called, is a simple compartmental model, first introduced by Ross. It is included in `exDE` as a base model. 
 
+**Parameters**
+
++ $r$ is the rate infections clear
+
++ $b$ is the fraction of bites by infective mosquitoes that transmit parasites and cause an infection. 
+
++ $c$ is the fraction of bites on an infectious human that would infect a mosquito. 
+
+**Dynamics**
+
 The model defined herein is defined in two parts. To model exposure and infection (*i.e.* the conversion of eir into foi), we assume that the fraction of infective bites that cause an infection is $b$. The dynamics are given by: 
 
 $$
 \frac{dX}{dt} = h (H-X)-rX
 $$
 
+**Net Infectiousness**
+
 True prevalence is:
 
 $$x = \frac{X}{H}.$$ 
 
 In our implementation, net infectiousness is linearly proportional to prevalence:
 
 $$c x.$$
-In this implementation, there is no immunity, but we assume a constant fraction of all infectious bites cause an infection. 
 
-**Parameters**
-
-+ $r$ is the rate infections clear
-
-+ $b$ is the fraction of bites by infective mosquitoes that transmit parasites and cause an infection. 
-
-+ $c$ is the fraction of bites on an infectious human that would infect a mosquito. 
+*Exposure and Infection*
 
+In this model family, there is no immunity. The model assumes that a constant fraction of all infectious bites, $b$, cause an infection. 
 
-## $\dagger$ 
 
-**References** 
+## References
 
 1. Ross R. Report on the Prevention of Malaria in Mauritius. London: Waterlow; 1908.
 2. Ross R. The Prevention of Malaria. 2nd ed. London: John Murray; 1911.