Great point. Most tourists check with their doctors before visiting a foreign country to see what diseases are particularly prevalent and get vaccinated against them before travelling. This plus the fact that the number of tourists is very low in comparison to actual residents means herd immunity may probably not be affected that much. However the article does make a good point that herd immunity could be influenced with a large number of visitors. I personally believe herd immunity is very important, and stops the spread of many infectious diseases.