The immune system is a complex system of interaction of cells having purpose is to detect foreign substances which are referred to as antigens. The immune system provides protection from infection. Due to variety of forms, immune responses need to deal with every type of infection. Vaccination is the most effective method of prevention of diseases; immunity due to vaccination is responsible for eradication of smallpox and many diseases such as polio, measles, and tetanus from the world. Some vaccines are administered after the patient has contracted a disease. Vaccines are given after exposure to smallpox, in first three days, are found to attenuate the disease, and vaccination up to a week after exposure offers some protection from disease. The first rabies immunization was given by Louis Pasteur to a child after the child was bitten by a dog found to have rabies .It has been found that, in people with uncompromised immune systems, four doses of rabies vaccine for 14 days, wound healing, and treatment of the bite with rabies immune globulin, starts as soon as possible after exposure, is useful in prevention of rabies in humans.

Epidemiology is the study of the causes, statistics and health effects of disease conditions in a particular population. Epidemiology shapes policy decisions and proof-based actions through identification of risk factors for disease and targets for preventive health. Epidemiologists with the help of study design, collection, and statistical analysis of data, and by interpreting and disseminating results which include peer review and occasional systematic review. Epidemiology helps to develop methodology to be used in clinical research studies, and, to a lesser extent, basic research in the biological sciences. Epidemiological studies involve an introduction, entire historical background, surveillance data sources, laboratory diagnosis, the biological characteristics of the organism, mechanisms and routes of transmission, pathogenesis and immunity, host response, and finally prevention, control and treatment. Emerging molecular methods are vast for future efforts. Traditional case control and cohort studies will be needed to define the action of such pathogens in disease causality.

Pathology is a branch of medical science which involves the examination of organs, tissues, and bodily fluids in order to diagnose a disease. It is a major field in modern medical practice and health care. Pathology outlines the condition of disease and treatment by using laboratory techniques. This is used in the diagnosis, treatment of an increasing range of clinical conditions. Pathology investigations are an integral part of the clinical treatment and diagnostic process with studies indicating 70-80 per cent of all health care decisions affecting diagnosis or treatment depends on pathological investigation. Clinical Pathology is a medical specialty that is concerned with the diagnosis of disease based on the laboratory analysis of body fluids. Molecular pathology in another branch of pathology imparting a few practices with drug development, treatment strategies. It is a discipline of pathology which is the study of ailments by examining molecules of organs, tissues or body fluids. It is multi-disciplinary and concentrates primarily on minute parts of diseases.

Nosocomial Infection involves infection which is contracted from the surrounding or through staff of a hospital or healthcare centers. It can be contracted in the hospital environment, nursing home, rehabilitation centers or other clinical settings. Nosocomial Infection is transferred to the susceptible patient in the clinical setting by a number of ways. Health care personnel can spread infection, contaminated equipment, bed linens, or air droplets. The infection can be spread from the outside of the hospital environment, from another infected patient, staff that may be infected, or the source of the infection cannot be detected. In some cases the microorganism originates from the patient's own skin flora, or opportunistic after any surgical process or other procedures that compromised the protective skin barrier. Although the patient itself has contracted the infection from their skin, the infection is still considered nosocomial since it develops in the same hospital. Hospitals have sanitation protocols regarding staff uniforms, sterilization of equipment, washing of hands, and other preventive measures.

Infection control is concerned with the prevention of nosocomial or clinic-related infection. It is often under recognized, is a part of the infrastructure of health care. Infection control and hospital epidemiology are related to public health practice, within a particular health-care delivery system. Anti-infective agents are usually antibiotics primarily antibacterial, antivirals, antifungals and antiprotozoal are available to eradicate infections. Infection control includes factors related to the spread of infections in the hospitals or healthcare centres include prevention through hand hygiene, cleaning/disinfection/sterilization, vaccination or surveillance and investigation of spread of infection in a health-care setting and management. Sterilization is a process to kill all microorganisms. Sterilizers are of heat, steam, or liquid chemicals. Disinfection uses liquid chemicals and at room temperature to kill microorganisms. Ultraviolet light can be used to disinfect the rooms of patients infected after discharge. Disinfection is less effective than sterilization because disinfection does not kill bacterial spores. Personal protective equipment (PPE) is some clothing or equipment to be worn by a staff for protection against hazard. The hazard in a healthcare setting is spilled blood, saliva, or other body fluids or aerosols that carry infectious materials such as Hepatitis C, HIV, or other blood borne or body fluid pathogen.

Parasitology is the study of parasites and their host’s relationship. Medical parasitology deals with the parasites infecting humans, the diseases caused by them, clinical pathology and the response generated by humans against them. It is also concerned with the various methods of their diagnosis, treatment and finally their prevention & control. A parasite is an organism that lives on or within other organism called the host. Medical parasitology also involves drug development, epidemiological studies and study of zoonosis. it is an interdisciplinary field, influenced by microbiology, biochemistry, immunology and other life sciences field. Parasites survive in the host of a highly immune-potent immune system. Vector-borne infections are parasitic infections transferred by the bite of infected arthropod species, such as mosquitoes, ticks, bugs, sandflies, and blackflies. Arthropods are cold-blooded and sensitive to climate factors. Resistant parasitic worms are having resistance against other infectious agents too, although the mechanisms of resistance in protozoan infections are not yet understood. About 50% of the positive malaria cases are from P. vivax. Current developments in new diagnostic tools, have opened new aspects for a huge improvement in parasite detection. There are some parasitic infection like cryptosporidiosis, there is no effective chemotherapy.

A branch of biology that concerns with the study of fungi, with their genetic and biochemical properties, their taxonomy and their use to humans as a source of wine, cheese,edible mushrooms and their harmful effects such as toxin or infection. Fungi and other organisms recognized as fungi, such as oomycetes and myxomycetes (slime molds), often are economically essential, as some of them cause diseases in animals such as histoplasmosis. Current research states that mushrooms may have hypoglycemic, anti-cancer, anti-pathogenic and immune system-enhancing activity.Food spoilage caused by fungi and yeasts can be more significant, particularly in a number of key food groups, those that are acidic in nature or have low moisture content. Mycotoxicology is another branch of mycology that focuses on study of toxins produced by fungi, called as mycotoxins. Fungi are having an important role in the ecosystem where they break down and decompose dead plants and animals (SAPROPHYTES). Some fungi are there which attack living organisms and are very destructive resulting in losses to the forestry industry.

Viral infections caused by viruses are very common, among them the common cold, influenza, rabies, measles, many forms of diarrhea, hepatitis, Dengue fever, polio, smallpox and AIDS Herpes simplex causes cold sores and genital herpes. The study of the ways by which viruses cause disease is termed as viral pathogenesis. The extent to which a virus causes disease is its virulence. When a virus enters into the host immune system, virus produces specific antibodies which bind to the virus and neutralize its infectiveness or destroy it. Presence of antibody in blood serum is used to detect whether a host has been exposed to a given virus in the past or not, with the help of tests such as ELISA. Vaccinations protect against viral diseases, by enhancing the production of antibodies. Monoclonal antibodies, specific to the virus, are used for detection, as in fluorescence microscopy. The use of viruses as gene vectors is done in the gene therapy of genetic diseases. Also in phage therapy, the use of bacteriophages to combat bacterial diseases was a great research topic before the use of antibiotics.

Bacterial diseases comprise all infections whose vital causative organism is bacteria. Bacterial pathogenesis is the process by which bacteria infect and cause disease in a host. Not all bacteria are pathogens and have the ability for pathogenesis. Bacteria also reside in host without providing harm and provide immune response to the host by invading foreign bacteria from invading the host. Bacteria produce two types of toxins called Exotoxins and Endotoxins. Bacterial toxins are soluble and cell-associated, can be transported by blood and lymph and can cause many cytotoxic effects at tissue sites remote from the original point of attack. Bacterial skin infections include Impetigo, Erysipelas, and Cellulitis and many more. Urinary tract infection is mainly caused by bacteria Escherichia coli. Typhoid is caused by enteric bacteria Salmonella typhi. Bacterial infections may be treated with antibiotics, which can be classified as bactericidal when they kill bacteria and bacteriostatic when they just prevent bacterial growth.

Antimicrobials are agents killing or inhibiting the growth of microorganisms and are grouped as antibacterial, antivirals, antifungals and antiparasitics. Antibacterial used to treat bacterial infections are termed as antibacterial, treating virus are termed as antivirals and hence antifungals and antiparasitics are respectively for fungi and parasites. The toxicity of antibacterial to humans and other animals is generally low. Antimicrobial pesticides control growth of microbes by the use of disinfection, sanitation, or reduction of development and to protect industrial processes or systems, surfaces, water, or other chemical substances from contamination, spoiling caused by bacteria, viruses, fungi, protozoa, algae.

Identification of an infectious agent for a particular illness can be as done as clinical presentation; such as in gastrointestinal disease and skin infections. Diagnosis of infectious disease is mostly done by consultation of patient's medical history and a physical examination. Microbiology laboratory plays a vital role where diagnosis and final interpretation of causative agents are done by experts. Microbiological culture is the first method for isolation of infectious diseases in the laboratory followed by biochemical and by advanced serological assays and through polymerase reactions. Experts often make treatment as a step of prevention to the patient’s physician depending on the strain of microbe and its antibiotic resistances.

Antimicrobial chemotherapy is the clinical use of antimicrobial agents in the treatment of infectious disease. The positive of outcome of antimicrobial therapy with antibacterial depends on several factors. These include host defense mechanisms, site of infection, and the pharmacokinetic and pharmacodynamics of the antibacterial. A bactericidal activity of antibacterial depends on the bacterial growth, often requires undergoing metabolic activity and bacterial division. Findings are based on laboratory studies, and in hospital settings it is used to eliminate bacterial infection. Since the activity of antibacterial depends on its concentration, in vitro characterization of antibacterial activity commonly involves the minimum inhibitory concentration and minimum bactericidal concentration of antibacterial. Antimicrobials used to treat bacterial infections termed as antibacterial chemotherapy, subsequently for fungal, protozoan and viral infections are antifungal, antiprotozoal and antiviral chemotherapy.

Antimicrobial resistance is a vital issue that leading to millions of deaths every year. Infections became completely untreatable due to antimicrobial resistance. All microbes develop resistance such as fungi develop antifungal resistance, viruses develop antiviral resistance and hence protozoa are developing antiprotozoal resistance and ultimately bacteria developing antibiotic resistance. Bacterial antibiotic resistance poses the largest threat to infection prevention in masses. Antibiotics should only be used when only it is essential and only when prescribed by health professionals. To prevent this issue of antimicrobial resistance awareness should be made to use only Narrow-spectrum antibiotics rather than broad-spectrum antibiotics accurately target specific organisms. Resistant genes transfer from non-pathogenic to pathogenic one, leading to clinically significant antibiotic resistance in host.

Microbial biochemistry involves biochemical reactions of microbial growth and modes and mechanisms of pathogenesis in causing infection in host. It is the study of microbial growth, microbial metabolism and microbial cell structure deal with structures, functions and interactions of biological macromolecules like proteins, nucleic acids, carbohydrates and lipids, which provide the structural entity and basis of  functions associated with life. Biochemical study of microbes is essential in the mechanisms of their action. Post genomic analyses of broad range of host plasmids, functional replication, maintenance of mechanisms, conjugation systems and regulatory network integrating plasmid functions plays a vital role in metabolism of microbes. Nutritional deficiencies of cultural medium can also be determined by the chemical methods.

Infectious diseases are caused by microorganisms like bacteria, viruses, parasites or fungi which can be spread from one person to another directly or indirectly. Infectious diseases can be classified as bacterial or viral, zoonotic, parasitic or fungal infections depending upon the causative agents. Most of infections do not result in death of the host and the infecting organism is removed after the symptoms of the disease have waned. The process needs immune mechanisms to kill or inactivate the inoculum of the pathogen. Specific acquired immunity against infectious diseases may be mediated by antibodies or T lymphocytes. The immune response of microbes causes symptoms like high fever and inflammation, and has the potential to be devastating than direct damage caused by a microbe. Phylodynamic models aid in finding the epidemic and pandemic origins.

Health science involves applied sciences that include the use of science, technology, biomedicine in healthcare. Diagnostics methods like physical examination, medical history, medical imaging cover the basics of first diagnosis of illness which follow further laboratory diagnosis of patient blood for any infectious agents. Health care is considered in the application of the knowledge which acquires through health science. Health care can be provided through a variety of fields, including pharmacy, dentistry and medicine field, also it involves treatment of illness and prevention of illness.

Host pathogen interaction is the interaction which takes place between a pathogen and host. Pathogens also interact with host without causing any disease which is usually termed asymptomatic infection. Microbes can be both hosts and pathogens, e.g. bacteria can infect animals or plants but are themselves infected by viruses. Studies of bacterial pathogenesis can lead to identification of molecular differences between pathogenic and nonpathogenic microbes. Virulence of microbes depends on host factors, for example the pathogenicity of avirulent microbes in immunocompromised hosts and the lack of pathogenicity of virulent pathogens in immune hosts.

Medical microbiology is the study of prevention, diagnosis and treatment of infectious diseases. Medical microbiology studies various applications of microbes for the betterment of health and prevention of epidemics and outbreak of diseases. Four kinds of microorganisms causing infectious disease are bacteria, fungi, parasites and viruses and infectious protein called a prion. Detailed identification techniques usually used in laboratories are microbial culture, microscopy, biochemical tests and genotyping. Medical microbiologists make treatment recommendations to the physician based on the report of strain of microbe and antibiotic resistances, the site of infection, the potential toxicity of antimicrobial drugs and any drug allergies of the patient.