Ⅰ. Introduction

infectious diseases were the most serious health problem in the world until the beginning of the 20th century, when chronic degenerative diseases began to dominate this scenario in developed countries.

According to William H. McNeill: "the role of infectious diseases in all the factors which have defined the course of the historical evolution of human civilization has been underestimated, and considers that this role was as important as that of the determinants economic and military. "

The appearance of infectious diseases, mainly in the form of epidemics, with these explosive and unpredictable characteristics are still today a cause of fear, insecurity and panic, as we could clearly see during the recent epidemic of SARS.

Ⅱ. Definitions

Infection is the term that defines the entry and development of an infectious agent in a human or animal body, whether or not it turns into a disease.

According to the National Institutes of Health, infection is the invasion and growth of germs in the body. Germs can be bacteria, viruses, yeasts, fungi or other microorganisms. Infections can start anywhere in the body and can spread throughout the body.