Infectious human diseases are caused by pathogens, of which there are six types: bacteria, viruses, protists (unicellular organisms), fungi (unicellular/multicellular organisms), parasitic worms (multicellular organisms), and prions (proteins). Pathogens are unique to the environment in which they evolved, with only a few having a worldwide distribution. The migration of humans since the ice age has nevertheless, served to transfer many pathogens and their vectors to new locations. Pathogens can be acquired from the environment, from an intermediate vector, or passed directly from person to person. Those passed directly from person to person are placed in a special class referred to as communicable disease-causing organisms. The effects of pathogens can be mild, such as the common cold, or virulent such as the Plague. This book focuses on the biology of virulent pathogens, the diseases they cause, the reaction of the human immune system, and the application of medical science. Some diseases are, or have, the ability to become endemic. The defining feature of an endemic disease is that it can always be found in the population that lives in a defined geographic area. Endemic diseases are not necessarily present at high levels and may be relatively rare. However, under certain circumstances, endemic diseases have the potential to produce an epidemic that could turn into a pandemic, depending on the reservoir(s) for the disease and its vector(s). The diseases discussed in this book all have (or had in the case of Smallpox) a high degree of “pathogenicity.” Pathogenicity is an organism’s ability to cause disease as determined by its “virulence” factors. Virulence is defined as a pathogen’s ability to cause damage to a host. The pathogens chosen for inclusion in this book are all virulent, and some are rapidly increasing their presence in the global milieu. A selection of representative specimens from each family of pathogens, as outlined above, is included in this book. The targets of the pathogens described in this book are the “eukaryotic” cells found in human bodies. In order to fight off disease-causing pathogens, the body marshals the cells of the immune system to fight off the invading pathogens. Medical science also offers vaccines and drugs to help defeat potential invaders. Infectious diseases break down into the following subtypes: Enteric diseases are caused by micro-organisms such as viruses, bacteria, and parasites, which produce intestinal illness. They are frequently transmitted via a fecal-oral route. Cholera, Polio, Typhoid, and Typhus are good examples. - Respiratory diseases are caused by organisms such as viruses, bacteria, and fungi, which affect the lungs and throat. The organisms can be spread by coughing, sneezing, or face-to-face contact. Tuberculosis, Coronavirus, Influenza, Valley Fever, and Pneumonia are good examples. - Respiratory-transmitted diseases are spread by coughing, sneezing, and face-to-face contact. They do not, however, transfer a respiratory disease. Ebola, Measles, Mumps, and Pneumonic Plague are good examples. - Sexually transmitted diseases (STDs) are caused by micro-organisms such as viruses or bacteria, which are exchanged through sexual contact, particularly vaginal, oral, or anal sex. Syphilis, Gonorrhea, and Aids are good examples. - Vector-borne and zoonotic diseases caused by viruses, bacteria, or parasites, which are transmitted to humans from animals or insects. Yellow Fever, Sleeping Sickness, Malaria, and Zika are all good examples. - There are also a few miscellaneous methods of transmission, such as contracting parasites while walking barefoot on infected soil. Noncommunicable diseases (NCDs) such as Cancer, or chronic diseases such as Multiple Sclerosis or Diabetes, which tend to be of long duration and are the result of a combination of genetic, physiological, and environmental, are not discussed in this Book.