I tried with following but it didn't go well

Can somebody please help me or give me some advice how to begin? Thanks in advance.

Code: Select all
Code, edit and compile here:
\begin{tikzpicture}[node distance=2 cm,auto,>=latex']\node [int, pin={[init]above:$\mu$}] (a) {$S$};\node (b) [left of=a,node distance=2cm, coordinate] {a};\node [int, pin={[init]above:$\mu$}] (c) [right of=a] {$I$};\node [coordinate] (end) [right of=c, node distance=2cm]{};\path[->] (b) edge node {$a$} (a);\path[->] (a) edge node {$v$} (c);\draw[->] (c) edge node {$p$} (end) ;\end{tikzpicture}