[Lonesome in Massachussets]

Not particularly hardy, street bums have an appalling mortality rate. Their bodies are already overstressed by the their life style. The last thing they need is a virus.

[NobleFree]

As a vulnerable population they've survived every infectious disease that has come along, with minimal if any medical care and living in unsanitary conditions; natural selection ensures that live bums are the hardiest people around.

street bums have an appalling mortality rate.

Which supports my statement that the ones who survive are hardy.