\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\tikzstyle{node}=[circle,minimum size=1cm,font=\bf\sf];
\tikzstyle{inactive}=[fill=black!20];
\tikzstyle{active}=[fill=blue!20];
\tikzstyle{activepath}=[line width=1mm];
\tikzstyle{inactivepath}=[line width=0.25mm];
\node[node,active] (A) at (0,0) {A};
\node[node,inactive] (B) at (2,-1.5) {B};
\node[node,active] (C) at (1.5,1.5) {C};
\node[node,inactive] (D) at (2,4) {D};
\node[node,inactive] (E) at (6.5,4.5) {E};
\node[node,active] (F) at (4,1.5) {F};
\node[node,inactive] (G) at (4.5,-1.5) {G};
\node[node,inactive] (H) at (5,-3.5) {H};
\node[node,inactive] (I) at (6.5,-5.5) {I};
\node[node,inactive] (J) at (7,-1.5) {J};
\node[node,active] (K) at (6.6,1) {K};
\node[node,active] (L) at (9,2.5) {L};
\node[node,inactive] (M) at (10,0) {M};
\node[node,inactive] (N) at (11,-2) {N};
\node[node,inactive] (O) at (11,-7) {O};
\node[node,inactive] (P) at (13,-2) {P};
\node[node,active] (Q) at (12,3) {Q};
\draw[activepath] (A) -- (C);
\draw[activepath] (C) -- (F);
\draw[activepath] (F) -- (K);
\draw[activepath] (K) -- (L);
\draw[activepath] (L) -- (Q);
\draw[inactivepath] (A) -- (B);
\draw[inactivepath] (B) -- (G);
\draw[inactivepath] (G) -- (F);
\draw[inactivepath] (G) -- (H);
\draw[inactivepath] (G) -- (J);
\draw[inactivepath] (J) -- (K);
\draw[inactivepath] (J) -- (M);
\draw[inactivepath] (J) -- (N);
\draw[inactivepath] (M) -- (L);
\draw[inactivepath] (M) -- (N);
\draw[inactivepath] (N) -- (P);
\draw[inactivepath] (P) -- (Q);
\draw[inactivepath] (P) -- (O);
\draw[inactivepath] (H) -- (I);
\draw[inactivepath] (I) -- (N);
\draw[inactivepath] (I) -- (O);
\draw[inactivepath] (D) -- (C);
\draw[inactivepath] (D) -- (E);
\draw[inactivepath] (E) -- (K);
\draw[inactivepath] (E) -- (Q);
\end{tikzpicture}
\end{document}