Airborne diseases are diseases that you can catch merely by breathing. These diseases are spread when those who are infected sneeze, cough, talk, or even exhale, releasing throat and nasal secretions into the air.
While the diseases can then land on and contaminate surfaces, they can also hang in the air, so that others nearby — including both humans and animals — can inhale them and become infected.
Airborne diseases tend to spread more easily than other types of diseases, and, thus, are harder to control.