p <- c(s=1,d=1,c=1,h=1)

size <- 5 #inch

pdf("virulenceA.pdf",width=size,height=size)
par(mar=c(0.1,0.1,0.1,0.1),xaxt="n",yaxt="n",ann=F,xaxs="i",yaxs="i")
with(as.list(p),curve(c*v/(d+v),from=0,to=10,xname="v",col="red"))
dev.off()

pdf("virulenceB.pdf",width=size,height=size)
par(mar=c(0.1,0.1,0.1,0.1),xaxt="n",yaxt="n",ann=F,xaxs="i",yaxs="i")
with(as.list(p),curve((c*v/(h+v))*(1/(d+v))*s/d,from=0,to=10,xname="v",col="red"))
dev.off()