Hypersensitivity reactions have been placed into four classes on the basis of their mechanism of immunologic injury. Type I or allergic reactions relate to the action of IgE and the release of powerful mediators, such as histamine from mast cells. Type II or cytotoxic reactions are created when IgG or IgM antibodies are misdirected to host cells. Type III or immune complex reactions are created when an excess of antigen–antibody complexes are deposited and followed by complement-mediated inflammation. Type IV reactions are cell-mediated and often called delayed-type hypersensitivity (DTH) because of the time delay in invoking the TH1 response. The hypersensitivity diseases include allergy, anaphylaxis, asthma, transfusion reactions, rheumatoid arthritis, and type 1 diabetes.

The immune response to infection is presented as two major components—innate immunity and adaptive immunity

These transmission-based precautions are divided into those directed at airborne, droplet, and contact routes. The airborne transmission precautions are for infections known to be transmitted by extremely small (<5 μm) particles suspended in the air

Sterilization is an absolute term. It means complete killing, or removal, of all living organisms from a particular location or material. It can be accomplished by incineration, nondestructive heat treatment, certain gases, exposure to ionizing radiation, some liquid chemicals, and filtration.

During infection—viral, bacterial, fungal, or parasitic—the host usually responds with the formation of antibodies, which can be detected by modification of any of the methods used for antigen detection. The formation of antibodies and their time course depend on the antigenic stimulation provided by the infection

but carry a higher risk of misinterpretation

These characteristics are sensitivity (the test’s ability to rule out [snout] a disease because there are few false-negative results and thus fewer cases missed) and specificity (the test’s ability to rule in [spin] or confirm an etiology because there are few false-positive results). Ideally, a test would have both excellent sensitivity and specificity, but traditional methods often involved a trade-off between the two, which only emphasizes the need to know the clinical question or reason for ordering a test. Molecular methods, however, tend to have improved sensitivity as well as specificity, which is dramatically so for viral etiologic diagnoses.

The best results are obtained when communication between the clinician and laboratory is optimal.

Epidemiology, the study of the distribution and determinants of disease, is critical for recognition and control of emerging infectious diseases. Emerging infectious diseases are those that are increasing in incidence, whether due to the appearance of a new agent, pattern of resistance, or geographic spread. Communicable diseases differ from noncommunicable diseases in their propensity to cause both endemic disease and pandemics. Infections may be clinically inapparent or may cause disease. Those with subclinical disease can be important propagators of the infectious agent. Transmission can be vertical (mother to fetus or infant) or horizontal (direct or indirect person to person). Routes of horizontal transmission include respiratory, salivary, eye, skin, genital, fecal-oral, bloodborne, and vector-borne or zoonotic. The propensity for epidemic spread of an infection depends on agent, host, and environmental factors. Surveillance is a key to recognition and thereby to control.

The prerequisites for propagation of an epidemic from person to person are: (1) a sufficient degree of infectivity to allow the organism to spread; (2) sufficient virulence for an increased incidence of disease to become apparent; and (3) sufficient level of susceptibility in the host population to permit transmission and amplification of the infecting organism

he virulence of an agent can be estimated as the number of fatal or severe cases per total number of cases. Incidence, the number of new cases of a disease within a specified period, is described as a rate in which the number of cases is the numerator and the number of people in the population under surveillance is the denominator. This is usually normalized to reflect a percentage of the population that is affected. Prevalence, which can also be described as a rate, is primarily used to indicate the total number of cases existing in a population at risk at a point in time. Diseases are more prevalent if they are especially common or less common but persist for a long time.

The disease index of an infection can be expressed as the number of persons who develop the disease divided by the total number infected

Many infections are transmitted by the respiratory route, often by aerosolization of respiratory secretions with subsequent inhalation by other persons. The efficiency of this process depends in part on the extent and method of propulsion of discharges from the mouth and nose, the size of the aerosol droplets, and the resistance of the infectious agent to desiccation and inactivation by ultraviolet light. The classic teaching is that in still air a particle 100 μm in diameter requires only seconds to fall the height of a room, a 10 m particle remains airborne for about 20 minutes, and smaller particles remain suspended even longer. When inhaled, particles with a diameter of 6 μm or more are usually trapped by the mucosa of the nasal turbinates, whereas particles of 0.6 to 5.0 μm attach to mucous sites at various levels along the upper and lower respiratory tract and may initiate infection. These “droplet nuclei” are most important in transmitting many respiratory pathogens (eg, M tuberculosis). Newer data suggest that humans with respiratory infections produce infectious aerosols comprising a wide range of particle sizes. SARS-CoV-2 coronavirus-2 is transmitted by both small and large particle aerosols; hence, surgical masks and physical “social” distancing (≥6 feet) are complementary approaches to preventing human-to-human transmission.

Various transmissible infections may be acquired from others by direct contact, indirectly through contaminated inanimate objects or materials, or by aerosol transmission of infectious secretions. Some infections, such as malaria, dengue, and chikungunya, involve an animate insect vector. These routes of spread are often referred to as horizontal transmission, in contrast to vertical or perinatal transmission—from mother to fetus or infant.

In general, organisms of high infectivity spread more easily, and those of greater virulence are more likely to cause disease than subclinical infection. The infecting dose of an organism also varies with different organisms and, thus, influences the chance of infection and development of disease.

Communicability of a disease in which the organism is shed in secretions may occur primarily during the incubation period.

Infection involves multiplication of the organism in or on the host and may be clinically inapparent, such as during the incubation period or latency (when little or no replication is occurring, eg, with herpesviruses). Disease occurs when the infection becomes clinically apparent, that is, there is evidence of injury to the host as a result of the infection. With many communicable organisms, infection is much more common than disease, and asymptomatic infected individuals are important for propagation of the infectious agent

Communicable infections can be endemic, present in the population at a low and constant level, or epidemic, present at a level of infection higher than that usually found in the community or population. With some infections, such as influenza, the infection can be endemic and persist at a low level from season to season; however, introduction of a new strain may result in epidemics, as illustrated in Figure 5–2. Communicable infections that are both widespread, for example, worldwide, and have high attack rates are termed pandemic.

Noncommunicable infections are those that are not transmitted from human to human and include: (1) infections related to the patient’s microbiota gaining access to a previously sterile site, such as peritonitis after rupture of the appendix; (2) infections caused by the ingestion of preformed toxins, such as botulism; and (3) infections caused by organisms found in the environment, such as clostridial gas gangrene. Some diseases transmitted from animals to humans (zoonotic infections), such as rabies and brucellosis, are not transmitted between humans, but others such as plague may be

An emerging disease is an infectious disease whose incidence has increased in the past two decades and/or that threatens to increase soon

a defense mechanism against infection and as a source of potentially pathogenic organisms

ntroduction of public health, immunization, and antimicrobial interventions.

public health, immunization, and antimicrobial interventions.

public health, immunization, and antimicrobial interventions.

viruses called virophages have been discovered that infect other viruses.

Microorganisms are the products of evolution, the biologic consequence of natural selection