As you probably already know, microbiology is the study of microscopic organisms. And these can be tiny single celled organisms like bacteria or colonies of cells like fungi, or even non-cellular entities like viruses. That need to live inside hosts cells in order to replicate. Microbes live in every ecosystem on earth and the earliest ones have been around for more than 3 billion years. The human microbiome is the collection of around 200 trillion bacteria that live on the skin, inside the mouth, and eyes and in the gastrointestinal tract of the average adult human. There are ten times as many microbes as there are cells in the adult human body. And many of these microbes are commensals, they live in peace with us or even help us to do things like break down nutrients in the digestive tract or produce vitamins that keep us healthy. But obviously, sometimes microbes can cause disease. And this happens in a variety of ways. In order to cause disease, microbes need to enter and colonize a part of the body they're not supposed to inhabit. Then they have to find ways of persisting there. Or replicating beyond their usual numbers. In order for these species to survive, they need to exit the host. And through all of these activities, microbes will evolve. Sometimes developing virulence factors that help them survive and potentially make them more pathogenic than they were before. The science and practice of immunology tries to understand the host immune defenses that either keep commensal microbes in check or actively defend the body against pathogenic ones. Physical barriers like skin and mucus membranes help keep pathogens out. And a variety of specialized cells help to protect the host from disease causing organisms. The first line cells in the immune system form part of the innate immune system. They detect and recognize potential pathogens. Even if they've never encountered them before. And they can then deploy a cascade of other body defenses. Including activating the adaptive immune system with the end goal of either eliminating the pathogen. Or in the case of commensals, finding a way to tolerate the existence of certain microbes in ways that don't result in disease. Through these encounters with microbes, the adaptive immune system will remember certain pathogens. And store an arsenal of deterrents in case that pathogen ever gets encountered again. Most of the time, the microbes in and on the host are successfully kept in check by the host immune system. But when this steady state is disturbed or if the immune system begins to behave inappropriately towards host cells, the result is a clinical presentation of disease. If the immune system becomes too weak, or too strong, or if the microbes become too aggressive, or go places they're not supposed to be in the body, the equilibrium is disturbed, and physicians need to figure out what went wrong. By understanding the characteristics of different microbes, and how the host immune system usually reacts to them. Physicians can recognize collections of signs and symptoms and integrate that with other details from the history and physical exam, to lead to a list of likely diagnoses. Which can then be confirmed in many cases, with the help of things like lab tests and imaging. Understanding the relevant microbiology and immunology, as it applies to each patient, helps the physician initiate the right therapy. For example, choose the right antibiotic for a specific type of bacteria. Or recognize when the immune system may itself be causing damage to the body, and the physician can then try to mediate. Knowing when these diseases are most likely to occur, how different microbes are spread, and how they cause disease. This knowledge can help with prevention efforts, both at the level of the individual patient and on the population level, when we start thinking about the epidemiology of a disease. Using a framework of the this kind can help physicians approach a variety of different diseases in an organized and systematic way. By trying to understand the clinical presentation of a patient as a result of the interplay between microbes and the host immune system, we'll be much more likely to succeed in managing the disease