ABSTRACT
A disease is any abnormal condition of the body that causes discomfort, anxiety, dysfunction, or distress to the person affected. This term is used broadly to include injuries, disabilities, symptoms, syndromes (set of symptoms), unusual behaviour, and abnormal structures and functions. More specifically, this term is used to describe an atypical condition in the living organism that interferes with the normal bodily function of the organism resulting in symptoms (characteristics observed or felt by the patient), signs (observed or measured by others), and ill-health. Disease is also referred to as morbidity. Of interest, the term 'infection' is used to describe when a pathogen invades a host, while 'disease' is described when the invading pathogen alters normal body function. There are different kinds of human diseases (Table 1). Among them, diseases caused by agents such as viruses, bacteria, protozoa, fungi and metazoa (usually helminths) are called 'infectious diseases'. Except metazoa, other pathogens are very small (invisible to naked eye) and are called microbial pathogens and their diseases are referred to as 'microbial infectious diseases' or simply 'microbial diseases' (Fig. 1).
