title: set the starting node for spreading a virus in a network
body: <p>I'm working on finding influential nodes in complex networks using R programming. I want to use degree centrality which means the number of neighbors a node has in a graph. I have a graph and the degree centrality of each node. Now I want to know how many nodes will be infected within a specified time when we start to spread the virus from each node. according to my studies I should use SIR(susceptible, infected, recovered) epidemic model which I found in "igraph" package, the problem is that I can't specify the starting node. It seems that this functions works based on SIR equations:  </p>
  
  <pre><code>s'= -(beta)SI
  I' = (beta)SI - (gamma)I
  R' = (gamma)I
  </code></pre>
  
  <p>where beta is infection parameter and gamma is recovery parameter.
  here is the igraph SIR code:</p>
  
  <pre><code>function (graph, beta, gamma, no.sim = 100) 
  {
  if (!is_igraph(graph)) {
      stop("Not a graph object")
  }
  beta &lt;- as.numeric(beta)
  gamma &lt;- as.numeric(gamma)
  no.sim &lt;- as.integer(no.sim)
  on.exit(.Call("R_igraph_finalizer", PACKAGE = "igraph"))
  res &lt;- .Call("R_igraph_sir", graph, beta, gamma, no.sim, 
      PACKAGE = "igraph")
  class(res) &lt;- "sir"
  res
  }
  </code></pre>
  
  <p>It seems that most of the work is being done in "R_igraph_sir" but I cant find such a function in that package.
  Is there any way to set the starting node?</p>