Infectious Disease

Colonization of a host by a microorganism implies that the microbe has become established and is multiplying on a body surface. If the microbe has a parasitic relationship with the host, then the term infection may be used. That is, a member of the normal flora is said to have colonized the host, but an organism capable of causing illness is described as having either colonized or infected it. Infection does not always lead to noticeable adverse effects. It can be subclinical or inapparent, meaning that symptoms either do not appear or are mild enough to go unnoticed, as in many cases of infection with polioviruses or the hepatitis A virus.

An infection that results in disease, a noticeable impairment of body function, is called an infectious disease. The disease causes characteristic symptoms and signs; symptoms are the effects of the disease experienced by the patient, such as pain and nausea, whereas signs are the effects of the disease that can be observed by examining the patient, such as rash, pus formation, and swelling.

One disease may leave a person predisposed to developing another. For example, various respiratory illnesses make a person more likely to develop pneumonia. The initial infection is called a primary infection; an additional infection that occurs as a result of the primary infection is called a secondary infection.


Some microorganisms and viruses are able to cause disease in otherwise healthy individuals; a microbe that is able to do this is referred to as a primary pathogen or, more simply, a pathogen. Diseases such as plague, malaria, measles, influenza, diphtheria, tetanus, and tuberculosis are caused by primary pathogens. A microbe that is able to cause disease only when the body's innate or adaptive defenses are compromised, or when introduced into an unusual location, is called an opportunistic pathogen, or opportunist. Opportunists may be members of the normal flora or common in the environment. For example, Pseudomonas species are ubiquitous and routinely come into contact with healthy individuals without harmful effect, yet they can cause fatal infections in individuals who have the genetic disease cystic fibrosis and also in burn patients (see figure 27.6). Ironically, as our health care systems improve, extending the life span of many patients through surgery and immunosuppressive drugs, diseases caused by opportunists are becoming more common. Also, many organisms not previously recognized as able to cause disease have now been shown to do so in some severely immunocompromised patients.

The term virulence is a quantitative term that refers to the degree of pathogenicity of an organism; it reflects both the proportion of infected people who have obvious symptoms and the relative severity of those symptoms. An organism that is described as highly virulent has more disease-promoting attributes than do other less virulent strains of the same species; the virulent organism is more likely to cause disease, particularly severe disease, than might otherwise be expected. Streptococcus pyogenes causes strep throat, for example, but certain strains are particularly virulent, causing diseases such as necrotizing fasciitis ("flesh eating disease"). ■ necrotizing fasciitis, p. 696

Characteristics of Infectious Disease

Infectious diseases that spread from one host to another are called communicable or contagious diseases. Some contagious diseases, such as colds and measles, are easily transmitted. The ease of spread of a contagious disease partly reflects the infectious dose, or the number of microbes necessary to establish an infection. For example, the intestinal disease shigellosis is quite contagious in humans because only 10 to 100 cells of a Shigella species need be ingested to establish infection; in contrast, sal-monellosis, which is not as contagious, requires ingestion of as many as 106 cells of Salmonella Enteritidis. The difference in these infectious doses reflects, in part, the ability of Shigella species to survive the acidic conditions encountered during passage through the stomach. Generally, the infectious dose is expressed as the ID50, an experimentally derived figure that indicates the number of microbes administered that resulted in disease in 50% of a population. ■ Shigella, p. 613 ■ Salmonella Enteritidis, p. 616

Course of Infectious Disease

The course of an infectious disease includes several stages (figure 19.2). The interval between introduction of an organism to a susceptible host and the onset of illness is the incubation period. The incubation period may vary considerably, from only a few days for the common cold, to several weeks for hepatitis A, to many months for rabies, and even years for Hansen's disease (leprosy). The length of the incubation period depends on the number of organisms encountered, the condition of the host, and many other factors.

A phase of illness follows the incubation period. During this period a person will experience the signs and symptoms of the disease. After the illness subsides, there is a period of convalescence, the stage of recuperation and recovery from the disease. Even though there is no indication of infection during the incubation and convalescent periods, many infectious agents may still be spread. Some individuals, called carriers, may harbor infectious agents for months or years and continue to spread pathogens, even though they themselves show no signs or symptoms of the disease. The impact of carriers on spread of disease will be discussed in chapter 20. ■ carriers, p. 489

Following recovery from infection, or after immunization, the host normally will have developed specific antibodies and immune cells that prevent reinfection with the same organism or virus. In most cases, the host is no longer susceptible to infection with that particular infectious agent.

19.3 Principles of Infectious Disease 463

Duration of Symptoms

Infections and the diseases they cause are often described according to the timing and duration ofthe symptoms (see figure 19.2):

■ Acute infections are characterized by symptoms that have a rapid onset but last only a short time; an example is strep throat.

■ Chronic infections develop more slowly and last longer, for months or years; an example is tuberculosis.

■ Latent infections are never completely eliminated; the microbe continues to exist in host tissues, often within host cells, for years without causing any symptoms. If there is a decrease in immune response, the latent infection may become reactivated and symptomatic. Note that the symptomatic phase of the disease may be either acute or chronic. For example, the infection caused by the varicella-zoster virus results in the characteristic rash and illness of chickenpox, an acute illness. The illness is halted by an effective immune response, leaving the host immune against reinfection. The virus, however, is not completely eliminated; it takes refuge in sensory nerves, held in check by the immune system. Later in life, if there is a decrease in immunity, infectious viral particles are produced, causing the skin disease called shingles (herpes zoster). In tuberculosis, the mycobacteria are often initially confined within a small area by host defense mechanisms, causing no symptoms; much later, if the host becomes immunocompromised, the bacteria may begin multiplying and destroying tissue, resulting in a chronic illness. Other diseases in which the causative agent becomes latent include cold sores, genital herpes, and typhus. ■ varicella virus, p. 546 ■ tuberculosis, p. 580

Distribution of the Pathogen

Infections are often described according to the distribution of the causative agent in the body. In a localized infection, the microbe is limited to a small area; an example is a boil caused by Staphylococcus aureus. In contrast, in a systemic or generalized

Incubation period



Figure 19.2 The Course of Infectious Diseases

Organism disappears after Diseases may be acute, chronic, or latent. illness; usually immunity to reinfection exists

Incubation period

Illness Illness persists or recurs over a long period

Incubation period



Latent period (no illness)

// //

Illness may recur

Illness may recur if immunity weakens

Days Months Years


Days Months Years


464 Chapter 19 Host-Microbe Interactions infection, the infectious agent is spread, or disseminated, throughout the body; an example is measles.

The suffix -emia means "in the blood." Thus, bacteremia indicates that bacteria are circulating in the bloodstream. Note that this term does not necessarily imply a disease state. For example, a person can become transiently bacteremic after vigorous tooth brushing. Toxemia indicates that toxins are circulating in the bloodstream. The organism that causes tetanus, for example, produces a localized infection yet its toxins circulate in the bloodstream. The term viremia indicates that viral particles are circulating in the bloodstream. Septicemia is an acute, life-threatening illness caused by infectious agents or their products circulating in the bloodstream.

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  • Abaalom
    What are the causative agent of infectious disease?
    4 years ago
    What are the infectous diseases, and there causative agent, incubation period?
    2 years ago
  • tapani
    What are the causatibe agent of communicable diseases?
    1 year ago
  • rowan
    What are the 50 causative agents of communicable diseases?
    8 months ago

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