It depends a slight bit on exactly how the disease is spread and how the symptoms show up; if it's not easily spread and if the symptoms show too early and too specific, then people will go into isolation and, thus, prevent a pandemic catastrophe. I will assume that it's airborne and that one becomes contagious well before any strong symptoms set in (i.e., it should start by looking like a common cold with just some minor coughing and sneezing to be unnoticed).

Anywhere with a lot of people gathered is the best place to go:

• International airport: A lot of people coming and going, as they leave to various places they make sure the disease is spread.
• Sports arenas: They can usually hold a couple of thousands of people, making it a good starting ground.
• Major events: Is the pope, president or the major rock star coming for a visit? That'll mean a lot of people gathering, potentially whom normally does not live together and, thus, will bring the disease with them home to other towns.
• Malls: not as large and wide spreading as the previous two locations, but still ample of people going there.
• Kindergartens: kids often get sick and they will spread it to their families, whom will pass it on to co-workers and friends, etc. Since kids often get sick, no one will suspect them to have anything worse than kid's diseases.

It depends a slight bit on exactly how the disease is spread and how the symptoms show up; if it's not easily spread and if the symptoms show too early and too specific, then people will go into isolation and, thus, prevent a pandemic catastrophe. I will assume that it's airborne and that one becomes contagious well before any strong symptoms set in (i.e., it should start by looking like a common cold with just some minor coughing and sneezing to be unnoticed). One extra important aspect is that one need to become contagious almost immediately, otherwise it won't really matter where you start as people will get sick, but leave the major area before they pass it on; if it becomes contagious slowly, then it's extra important that it goes unnoticed for a long time so it can spread to as many as possible.

If it does spread almost immediately (i.e., withing minutes to few hours), then anywhere with a lot of people gathered is the best place to go:

• International airport: A lot of people coming and going, as they leave to various places they make sure the disease is spread.
• Sports arenas: They can usually hold a couple of thousands of people, making it a good starting ground.
• Major events: Is the pope, president or the major rock star coming for a visit? That'll mean a lot of people gathering, potentially whom normally does not live together and, thus, will bring the disease with them home to other towns.
• Malls: not as large and wide spreading as the previous two locations, but still ample of people going there.
• Kindergartens: kids often get sick and they will spread it to their families, whom will pass it on to co-workers and friends, etc. Since kids often get sick, no one will suspect them to have anything worse than kid's diseases.

If it takes days before one becomes contagious, then it's best to give it to a few, select, persons he knows are going to travel through major airports and to infect well enough ahead them so that they are coughing viruses on everyone they meet. Kindergartens would work well too, but the other places mentioned will likely not work as well as people will be there too short time to make a major spread. If you are on a major event, then you might only meet a few dozen people close enough to spread anything to them, they will get sick but the rest of the people there will not catch the disease; it is also likely that among those dozen people, the infectious person will know most of them (one usually does go to even with friends, after all) and thus the spread will be localized.