I'd like make something like the pic below, but in horizontal format. How do I get the text boxes equidistant between susceptible and exposed? Relative positioning of nodes in TikZ uses coordinates. Isn't there a way to do with relative to an object?

\documentclass{article}
%\usepackage[latin1]{inputenc}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows}
\usetikzlibrary{calc,
arrows,decorations.pathmorphing,
backgrounds,fit,positioning,shapes.symbols,chains}
\usetikzlibrary{decorations.pathreplacing}
\begin{document}
% Define block styles
\tikzstyle{block} = [rectangle, draw, fill=blue!30,
text width=5em, text centered, rounded corners, minimum height=4em]
\tikzstyle{line} = [draw, -latex']
%\tikzstyle{curve}=[draw,(latex']
\begin{figure}[!ht]
\centering\makebox[\textwidth]{
\begin{tikzpicture}[node distance =4cm, auto,
comment/.style={
rectangle,
inner sep= 5pt,
text width=4cm, }]
% Place nodes
\node [block] (susceptible) {Susceptible};
\node [block, right of=susceptible] (exposed) {Exposed};
\draw [decorate,decoration={brace,amplitude=10pt},xshift=0pt,yshift=0pt]
(3.8,-2)--(-1,-2) node [black,midway,yshift=-0.6cm]
{ \emph{Environmental \& Human factors}
};
\node [block, right of=exposed] (infectious) {Infectious};
\draw [decorate,decoration={brace,amplitude=10pt},xshift=0pt,yshift=0pt]
(7.9,-2) -- (4.1,-2) node [black,midway,yshift=-0.6cm]
{ \emph{Variable immunity} };
\node [block, right of=infectious] (removed) {Removed};
\draw [decorate,decoration={brace,amplitude=10pt},xshift=0pt,yshift=0pt]
(12,-2) -- (8.2,-2) node [black,midway,yshift=-0.6cm]
{ \emph{Control parameters} };
\draw [decorate,decoration={brace,amplitude=10pt},xshift=0pt,yshift=0pt]
(0.1,1) -- (3.9,1) node [black,midway,yshift=0.6cm]
{\large Contact parameters};
\draw [decorate,decoration={brace,amplitude=10pt},xshift=0pt,yshift=0pt]
(4.1,1) -- (12,1) node [black,midway,yshift=0.6cm]
{\large Disease parameters};
\path [line] (susceptible) -- (exposed);
\path [line] (exposed) -- (infectious);
\path [line] (infectious) -- (removed);
\path[line,dashed] (removed.south) -- + (0,-.35) -- + (-12,-.35) -- (susceptible.south);
\end{tikzpicture}}
\smallskip
\label{fig:flowchart_disease_transmission}
\caption{Flowchart of fundamental disease transmission mechanisms}
\end{figure}
\end{document}
EDIT Looks like this and took me 2 hours (looks okay-ish):
In powerpoint, 5 mins but looks pants!
Still can't work out how to get an itemized list in the lower braces.


chains" examples. Also here – JLDiaz Mar 20 '13 at 16:36