Which three cities would be "optimal" for spreading a contagious agent throughout the world most effectively?

I have been watching zombie movies, reading zombie books and playing zombie games a lot lately and it’s gotten me to thinking about the likelihood of a viral pandemic of some sort. I think that if someone was to deliberately spread a highly infectious agent, they would have to choose a few really good targets for the initial infection. I have thought of three cities and general reasons why I think infecting them would cause a contagion to spread like wildfire throughout the world.

1. Shanghai – It is one of the biggest cities in the most populated country in the world. Infection would spread very fast through there simply because of this. It would then probably spread to all but the most remote and rural parts of China. It is also visited by non-Chinese citizens on a regular basis. This combined with the fact that it is so densely populated would mean that even if a tourist was to escape an obviously infected person, the chances are very good that they would run into someone who was not obviously infected, but still contagious and then possibly get on a plane and fly back home where they would infect even more people.

2. Las Vegas – This seems like an odd choice for a North American city to be infected, however, since people from all over the country and, at times, the world visit Las Vegas, I think it would be a prime target for taking out the North American continent and even the South American continent as well. I considered New York City, but the fact that the majority of it is on an island, the infection could be contained relatively quickly by simply bombing Manhattan. If anyone has compelling reasons as to why NYC would be a prime candidate, however, please let me know.

3. Tokyo – This also seems like an odd choice. However, even though Japan itself is not very large, it is densely populated and there are a number of international business men who have business in many different countries. Every airport would be an excellent breeding ground for infection and large airports with a multitude of international flights leaving every hour would be especially effective at spreading the disease.

So, Jellies, which three cities do you think would be the “optimal” sites for spreading an outbreak of this magnitude?

We can file this question under both curiosity and research because I am in the process of writing a book about a disease that killed most of humanity. It’s on the back burner for now, but I am still doing research on it every now and then.