Infectious diseases are disorders caused by organisms - such as bacteria, viruses, fungi or parasites.Many organisms live in and on our bodies. They're normally harmless or even helpful, but under certainconditions, some organisms may cause disease. Some infectious diseases can be passed from person toperson. Some are transmitted by bites from insects or animals. And others are acquired by ingestingcontaminated food or water or being exposed to organisms in the environment. The objective is tointroduce to students the essential principles for understanding various aspects of diseases. Most ofdiseases constitute the largest part of human pathology and are the primary cause of death. Hence,special importance is given to the study of such diseases. The book is intended to acquaint students ofvarious fields involved directly or indirectly with the major principles of human diseases. The book maybe helpful as well to practitioners and those engaged in medical research.