This is an important question. If you study the history of diseases, you will see that diseases spread rapidly at times causing much death and hardship. The reason for this quick spread is because there was a lack of quarantining people, who were contaminated. The most effective way to stop any spread is to isolate those who are infected from those who are not.
In view of this, the most appropriate steps to take is as follows.
First, you would want to locate what the disease is and how it spread. For instance, is the virus airborne, or does it need physical contact? Once this is determined, you can set a plan of action. Second, depending on the type of virus (and how it spreads), you will have to notify the people through all forms of media to get the word out. Third, from a more personal view, I would make sure that I wash my hands constantly. This might seem small, but it is rather effective. In addition, I would stay away from places where there are many people.
Finally, you will need to look for a cure and vaccine.