For each infected node on the network, find susceptible neighbors and
probabilistically spread the infection.
     
    infect(agents, neighbors, x0 = -2.556)
    Arguments
    
    
    
      | agents | 
      A data.table containing agent information.  | 
    
    
      | neighbors | 
      A list of data.tables, each containing the neighbors on
a different network.
Neighbor data.tables have columns: 
head: the head vertex
  
tail: the tail vertex
  
edge_type: the kind of network (e.g., "home", "social", or "work")
  
freq: the frequency of contacts on this edge (times per day the two
agents are in close contact).
  
strength: the intensity/closeness of the contact (e.g., health-care
visits are very strong, paying a cashier at a store would be rather
weak).
  
  | 
    
    
      | x0 | 
      The unmodified mean infection value..  | 
    
    
    Value
    A modified network graph after propagating the infections.
    Examples
    # ADD_EXAMPLES_HERE