If you are an epidemiologist……

Imagine if you are an epidemiologist,and you noticed many infectious disease cases in  the past days.What will you do?

The correct process should be :

First,collect the data of people involved in cases. The case means the health-related event, such as the outbreak of flu. We need to find the common characteristics of these people. The characteristics may be the genetic factors or the environmental factors. Are they special in origin? Are they exposed to certain environmental hazard? Did they have something in common, like eat some contaminated food or participate in a big event?
Then,we need to know about the modes of transmission.

                                                      Reference: Leon Gordis(2008),Epidemiology (4th edition),19.

As the graph shows,the disease can be transmitted in several ways. Health professionals would work to reduce the chances of the disease spreading.

Example: A person gets the flu, and the people around him are healthy.  Suddenly, this person sneezes on others. So, the virus could be transmitted to others by this unintended event.(Gordis)

One of the most concerning aspects of infectious disease is that harms can be spread from very few people to large groups of people. So, it is very important to avoid further transmission of the disease.

References:
Leon Gordis(2008),Epidemiology (4th edition),3-19.