Health topics
Infectious diseases
![](https://webarchive.library.unt.edu/web/20161228193555im_/http://www.who.int/sysmedia/images/topics/infectious_diseases.jpg)
![Boy with baby](https://webarchive.library.unt.edu/web/20161228193555im_/http://www.who.int/topics/infectious_diseases/ht_infectiousdisease.jpg)
Infectious diseases are caused by pathogenic microorganisms, such as bacteria, viruses, parasites or fungi; the diseases can be spread, directly or indirectly, from one person to another. Zoonotic diseases are infectious diseases of animals that can cause disease when transmitted to humans.