[danieltillett]

Yes it is really difficult to know how the two major forces are tipping the likelihood. We have a better understanding of how contagious disease is spread and on average much better immune systems because of better diets (the average person was starving when most plagues struck in the past). Balancing this is as you mention increased travel and much high population densities. You just have to see the spread of a contagious disease through a modern animal feedlot to know what this does. All of this makes it really hard to know which way the risk has changed.

My biggest fear is a pandemic Rhadinovirus since it is airborne, has a long incubation phase and leukaemia is the outcome [1]. Something like a human pathogenic Ateline or Saimiriine herpesvirus could spread through the human population without us knowing until it is too late.

Interestingly, Saimiriine herpesvirus has recently been linked to idiopathic pulmonary fibrosis [2]. All it would take is a more pathogenic strain of this virus and we would be in a lot of problems.

[1] http://en.wikipedia.org/wiki/Rhadinovirus

[2] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4050527/