The Center for Disease Control (CDC) defines infectious diseases as “illnesses caused by germs (such as bacteria, viruses, and fungi) that enter the body, multiply, and cause an infection.” Depending on the disease, there are different ways that it can be spread:
From one person to another
Through germs carried in air, water, food, or soil
By insects, ticks, or animals
Emerging diseases are infections that have increased recently or are threatening to increase in the near future. These infections could be
Newly discovered
Completely new to an area
Reappearing in an area
Caused by bacteria that have become resistant to antibiotics
https://www.cdc.gov/ncezid/who-we-are/about-our-name.html