How does global plane travel make us more susceptible to viruses?

Doesn't. You aren' more susceptible, but more exposed and therefore more likely to encounter a variety of pathogens. When you are enclosed in a metal tube for hours on end in close contact with people who may be incubating or suffering from a variety of illnesses you are infinitely more likely to be in touch with organisms like this than if you sitting on your porch drinking bourbon in rural kentucky.