Infections always have been and still are responsible for considerable morbidity and mortality worldwide. It was not until 1864 that Louis Pasteur concluded from his laboratory experiments that infectious diseases were caused by living organisms. Infections are mainly caused by micro-organisms such as bacteria, viruses and fungi.

The duration and severity of an infection depend on various factors: the person’s own immune responses and physical condition, the site of infection, and the virulence of the pathogen. An infection is termed an infectious disease when the site of infection, the symptoms and complaints are specifically defined. Often the disease-causing pathogens have been identified as well.

For optimum prevention and treatment it is essential to improve the understanding of causes, transmission, spread, and classification of specific infections.