The terms “infection” and “disease” are not synonymous. An infection results when a pathogen invades and begins growing within a host. Disease results only if and when, as a consequence of the invasion and growth of a pathogen, tissue function is impaired.
An infectious disease is a disease that is caused by a microorganism, such as a bacterium, virus, or parasite, that is not normally found in the body and is capable of causing infection. Our bodies have defense mechanisms to prevent infection and, should those mechanisms fail, to prevent disease after infection occurs. Some, but not all, infectious diseases are contagious, meaning they can spread from person to person. Other infectious diseases can spread from animals or insects to humans, but not from person to person.
Infectious diseases are responsible for more than 25% of 57 million annual deaths worldwide. 2/3 of 9 million pediatric deaths yearly are from infectious disease. Some of those deaths could be prevented with vaccines. Some infectious agents are easily transmitted (that is, they are very contagious), but they are not very likely to cause disease (that is, they are not very virulent).