It depends what you mean by easy! Some diseases are very highly infectious, but are relatively rare, so it would be difficult for you or me to get it. Other disease might be a bit less infectious, but much more widespread. I guess one of the most common and easiest ones to get is influenza, or the flu. Every year, about 2-3 million people get a severe case of the flu, with around about 500,000 people dying from it – so it can be very dangerous, as well as being relatively easy to catch.