Ezra Klein writing in vox.com explores the evolutionary basis for our "behavioral immune system." The basic point is that many of our reactions to illness and infection are rooted deeply in our collective psyche. Those reactions served us well in the past as we avoided diseases and those who carried them. But those reactions now serve us badly when disease can be spread widely by modern transportation. Check out his summary of the research on psychological reactions to illness.
Klein goes on to explain why the travel ban is a bad idea and why we need to stop Ebola at the source.