15.1: Characteristics of Infectious Diseases (2023)

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    learning objectives

    • Distinguish signs and symptoms of disease
    • Explain the difference between a communicable disease and a non-communicable disease
    • Compare different types of infectious diseases, including iatrogenic, nosocomial and zoonotic diseases
    • Identify and describe the stages of an acute infectious disease in terms of the number of pathogens present and the severity of signs and symptoms

    A disease is any condition in which the body's normal structure or functions are damaged or impaired. Physical injuries or disabilities do not qualify as a disease, but there can be many causes for the disease, including infection with a pathogen, genetics (as with many cancers or disabilities), non-infectious environmental causes, or inappropriate immune responses. Our focus in this chapter will be on infectious diseases, although when diagnosing infectious diseases it is always important to consider possible non-infectious causes.

    Clinical Focus: Part 1

    Michael, a 10-year-old boy in good health, attended a birthday party on Sunday with his family. He ate many different foods, but he was the only one in the family to eat the rare hot dogs served by his hosts. On Monday morning, he woke up feeling achy and nauseous and had a fever of 38 °C (100.4 °F). His parents, assuming Michael had the flu, forced him to stay home from school and limited his activities. But after 4 days, Michael started experiencing severe headaches and his fever rose to 40 °C (104 °F). Increasingly concerned, his parents finally decide to take Michael to a nearby clinic.

    Exercise \(\PageIndex{1}\)

    1. What signs and symptoms is Michael experiencing?
    2. What do these signs and symptoms tell us about the stage of Michael's disease?

    Signs and symptoms of the disease

    An infection is the successful colonization of a host by a microorganism. Infections can lead to disease, which causes signs and symptoms that result in a deviation from the host's normal structure or functioning. Microorganisms that can cause disease are known as pathogens.

    Signs of the disease are objective and measurable and can be observed directly by a clinician. Vital signs, which are used to measure basic body functions, include body temperature (typically 37 °C [98.6 °F]), heart rate (typically 60–100 beats per minute), respiratory rate (typically 12–18 breaths per minute) and blood pressure (usually between 90/60 and 120/80 mm Hg). Changes in any of the body's vital signs can be indicative of illness. For example, having a fever (a body temperature significantly higher than 37°C or 98.6°F) is a sign of illness because it can be measured.

    In addition to changes in vital signs, other observable conditions can be considered signs of illness. For example, the presence of antibodies in a patient's serum (the liquid portion of the blood that lacks clotting factors) can be observed and measured using blood tests and therefore can be considered a sign. However, it is important to note that the presence of antibodies is not always a sign of active disease. Antibodies can remain in the body long after an infection has resolved; in addition, they can develop in response to a pathogen that is in the body but is not currently causing disease.

    Unlike the signs, the symptoms of the disease are subjective. Symptoms are felt or experienced by the patient but cannot be clinically confirmed or objectively measured. Examples of symptoms include nausea, loss of appetite and pain. These symptoms are important to consider when diagnosing illnesses, but they are subject to recall bias and difficult to measure accurately. Some clinicians try to quantify symptoms by asking patients to assign a numerical value to their symptoms. For example, the Wong-Baker Faces Pain Rating Scale asks patients to rate their pain on a scale of 0 to 10. An alternative method of quantifying pain is to measure skin conductance fluctuations. These fluctuations reflect sweating due to skin sympathetic nerve activity resulting from the pain stressor.1

    A specific group of signs and symptoms characteristic of a particular disease is called a syndrome. Many syndromes are named using a nomenclature based on signs and symptoms or the location of the disease. The table \(\PageIndex{1}\) lists some of the commonly used prefixes and suffixes in naming syndromes.

    Table \(\PageIndex{1}\): Nomenclature of symptoms
    quicklycellcytopenia: reduction in the number of blood cells
    hepat-from the liverhepatitis: inflammation of the liver
    -patiaillnessneuropathy: a disease that affects the nerves
    -emiaof the bloodbacteremia: presence of bacteria in the blood
    -that's itinflammationcolitis: inflammation of the colon
    -liseundoinghemolysis: destruction of red blood cells
    - tertumorlymphoma: cancer of the lymphatic system
    -osisdisease or abnormal conditionleukocytosis: an abnormally high number of white blood cells
    -dermaof the skinkeratoderma: a thickening of the skin

    Clinicians must rely on signs and ask questions about the patient's symptoms, medical history, and recent activities to identify a specific illness and potential causative agent. Diagnosis is complicated by the fact that different microorganisms can cause similar signs and symptoms in a patient. For example, an individual showing symptoms of diarrhea may have been infected with a wide variety of pathogenic microorganisms. Bacterial pathogens associated with diarrheal diseases includeVibrio cholerae,Listeria monocytogenes,Campylobacter jejuniand enteropathogenicEscherichia coli(EPEC). Viral pathogens associated with diarrheal diseases include norovirus and rotavirus. Parasitic pathogens associated with diarrhea includeGiardia lambliaeCryptosporidium minor. Likewise, fever is indicative of many types of infection, from the common cold to the deadly Ebola hemorrhagic fever.

    Finally, some diseases can be asymptomatic or subclinical, that is, they do not have noticeable signs or symptoms. For example, most individuals infected with the herpes simplex virus remain asymptomatic and are unaware that they have been infected.

    Exercise \(\PageIndex{2}\)

    Explain the difference between signs and symptoms.

    Disease Classifications

    The International Classification of Diseases (ICD) of the World Health Organization (WHO) is used in clinical fields to classify diseases and monitor morbidity (the number of cases of a disease) and mortality (the number of deaths due to a disease). In this section, we will introduce the terminology used by the ICD (and in the health professions generally) to describe and categorize various types of illnesses.

    Oneinfectious diseaseis any disease caused by the direct effect of a pathogen. A pathogen can be cellular (bacteria, parasites, and fungi) or acellular (viruses, viroids, and prions). Some infectious diseases are also communicable, meaning they can be passed from person to person through direct or indirect mechanisms. Some communicable infectious diseases are also considered contagious diseases, which means they are easily transmitted from person to person. Not all contagious diseases are equally contagious; the degree to which a disease is contagious often depends on how the pathogen is transmitted. For example, measles is a highly contagious viral disease that can be transmitted when an infected person coughs or sneezes and an uninfected person inhales droplets containing the virus. Gonorrhea is not as contagious as measles because transmission of the pathogen (Neisseria gonorrhoeae) requires close intimate (usually sexual) contact between an infected person and an uninfected person.

    Illnesses contracted as a result of a medical procedure are known asiatrogenic diseases. Iatrogenic illness can occur after procedures involving wound care, catheterization, or surgery if the wound or surgical site becomes contaminated. For example, an individual treated for a skin wound may acquire necrotizing fasciitis (an aggressive, "flesh-eating" disease) if bandages or other dressings become contaminated byClostridium perfringensor one of several other bacteria that can cause this condition.

    Diseases acquired in hospital environments are known asnosocomial diseases. Several factors contribute to the prevalence and severity of nosocomial illnesses. First, sick patients bring numerous pathogens into hospitals, and some of these pathogens can easily be transmitted through improperly sterilized medical equipment, sheets, call buttons, doorknobs, or by doctors, nurses, or therapists who do not wash their hands beforehand. touch a patient. Second, many hospitalized patients have weakened immune systems, making them more susceptible to infections. Furthermore, the prevalence of antibiotics in hospital settings can select for drug-resistant bacteria that can cause very serious and difficult-to-treat infections.

    Certain infectious diseases are not transmitted directly between humans, but can be transmitted from animals to humans. Such a disease is called a zoonotic disease (or zoonosis). According to the WHO, a zoonosis is a disease that occurs when a pathogen is transferred from a vertebrate animal to a human being; however, the term is sometimes defined more broadly to include diseases carried by all animals (including invertebrates). For example, rabies is a viral zoonotic disease transmitted from animals to humans through bites and contact with infected saliva. Many other zoonotic diseases rely on insects or other arthropods for transmission. Examples include yellow fever (transmitted by the bite of mosquitoes infected with the yellow fever virus) and Rocky Mountain spotted fever (transmitted by the bite of ticks infected withRickettsia rickettsii).

    In contrast to communicable infectious diseases, a non-communicable infectious disease is not spread from one person to another. An example is tetanus, caused byClostridium tetani, a bacterium that produces endospores that can survive in soil for many years. This disease is normally only spread through contact with a sore on the skin; cannot be transmitted from an infected person to another person. Likewise, Legionnaires' disease is caused byLegionella pneumophila, a bacterium that lives inside amoebas in damp places like water cooling towers. An individual can contract Legionnaires' disease through contact with contaminated water, but once infected, the individual cannot transmit the pathogen to other individuals.

    In addition to the wide variety of non-communicable infectious diseases, non-infectious diseases (those not caused by pathogens) are an important cause of morbidity and mortality worldwide. Non-infectious diseases can be caused by a wide variety of factors, including genetics, environment, or immune system dysfunction, to name a few. For example, sickle cell anemia is an inherited disease caused by a genetic mutation that can be passed from parent to child (Figure \(\PageIndex{1}\)). Other types of non-infectious diseases are listed in Table \(\PageIndex{2}\).

    Table \(\PageIndex{2}\): Types of Non-Infectious Diseases
    inheriteda genetic diseasesickle cell anemia
    congenitalDisease that is present at birth or beforeDown syndrome
    degenerativeProgressive and irreversible loss of functionParkinson's disease (which affects the central nervous system)
    nutritional deficiencyImpaired bodily function due to lack of nutrientsScurvy (vitamin C deficiency)
    EndocrineDisease involving malfunction of glands that release hormones to regulate body functionsHypothyroidism – the thyroid does not produce enough thyroid hormone, which is important for metabolism
    neoplasticAbnormal growth (benign or malignant)Some forms of cancer
    idiopathicDisease whose cause is unknownIdiopathic juxtafoveal retinal telangiectasia (dilated and twisted blood vessels in the retina of the eye)
    15.1: Characteristics of Infectious Diseases (2)
    link to learn

    Lists of common infectious diseases can be found in the followingCenters for Disease Control and Prevention(CDC),World Health Organization(OMS) eInternational Classification of Diseasessites.

    Exercise \(\PageIndex{3}\)

    1. Describe how a disease can be infectious but not contagious.
    2. Explain the difference between iatrogenic disease and nosocomial disease.

    periods of illness

    The five periods of illness (sometimes called stages or phases) include the incubation, prodromal, illness, decline, and convalescence periods (Figure \(\PageIndex{2}\)). The incubation period occurs in an acute illness after the initial entry of the pathogen into the host (patient). It is during this time that the pathogen begins to multiply in the host. However, there are insufficient numbers of pathogen particles (cells or viruses) present to cause signs and symptoms of disease. Incubation periods can range from one or two days in acute disease to months or years in chronic disease, depending on the pathogen. Factors involved in determining the length of the incubation period are diverse and may include the strength of the pathogen, the strength of the host's immune defenses, the site of infection, the type of infection, and the size of the infectious dose received. During this incubation period, the patient does not realize that an illness is starting to develop.

    15.1: Characteristics of Infectious Diseases (3)

    The prodromal period occurs after the incubation period. During this phase, the pathogen continues to multiply and the host begins to show general signs and symptoms of the disease, which usually result from the activation of the immune system, such as fever, pain, pain, swelling or inflammation. Typically, such signs and symptoms are too general to indicate a specific disease. After the prodromal period comes the period of illness, during which the signs and symptoms of the disease are most obvious and severe.

    The period of illness is followed by the period of decline, during which the number of pathogen particles begins to decrease and the signs and symptoms of illness begin to diminish. However, during the period of decline, patients may become susceptible to developing secondary infections because their immune systems have been weakened by the primary infection. The final period is known as the convalescence period. During this phase, the patient usually returns to normal functions, although some illnesses can cause permanent damage that the body cannot fully repair.

    Infectious diseases can be contagious during all five periods of illness. Which periods of illness are most likely to be associated with the transmissibility of an infection depends on the disease, the pathogen, and the mechanisms by which the disease develops and progresses. For example, with meningitis (infection of the lining of the brain), the periods of infectivity depend on the type of pathogen causing the infection. Patients with bacterial meningitis are contagious during the incubation period for up to a week before the onset of the prodromal period, whereas patients with viral meningitis become contagious when the first signs and symptoms of the prodromal period appear. With many viral diseases associated with rashes (eg, chickenpox, measles, rubella, roseola), patients are contagious during the incubation period up to a week before the appearance of the rash. In contrast, with many respiratory infections (eg, colds, flu, diphtheria, strep throat, and whooping cough), the patient becomes contagious with the onset of the prodromal period. Depending on the pathogen, the disease and the infected individual, transmission can still occur during periods of decline, convalescence and even long after the signs and symptoms of the disease have disappeared. For example, an individual recovering from a diarrheal illness may continue to carry and excrete the pathogen in the stool for some time, posing a risk of transmission to others through direct or indirect contact (e.g., through contaminated objects or food). .

    Exercise \(\PageIndex{4}\)

    Name some of the factors that can affect the length of the incubation period for a given disease.

    Acute and chronic illnesses

    The duration of illness can vary greatly depending on the pathogen, the effectiveness of the immune response in the host, and any medical treatment received. For an acute illness, the pathologic changes occur over a relatively short period of time (eg, hours, days, or a few weeks) and involve a rapid onset of disease conditions. For example, the flu (caused by the Influenza virus) is considered an acute illness because the incubation period is approximately 1 to 2 days. Infected individuals can spread the flu to others for approximately 5 days after becoming ill. After approximately 1 week, subjects enter the decline period.

    For a chronic disease, pathologic changes may occur over longer periods of time (eg, months, years, or a lifetime). For example, chronic gastritis (inflammation of the stomach lining) is caused by the gram-negative bacteriaHelicobacter pylori.H. pyloriis able to colonize the stomach and persist in its highly acidic environment, producing the enzyme urease, which modifies the local acidity, allowing the bacteria to survive indefinitely.2Consequently,H. pyloriinfections can recur indefinitely unless the infection is cleared with antibiotics.3The hepatitis B virus can cause a chronic infection in some patients who do not clear the virus after an acute illness. A chronic hepatitis B virus infection is characterized by the continued production of infectious virus for 6 months or more after the acute infection, as measured by the presence of viral antigen in blood samples.

    In latent diseases, unlike chronic infections, the causative pathogen lies dormant for long periods of time without active replication. Examples of diseases that go into a latent state after acute infection include herpes (herpes simplex virus [HSV-1 and HSV-2]), chickenpox (varicella-zoster virus [VZV]), and mononucleosis (Epstein-Barr virus [EBV]). . HSV-1, HSV-2, and VZV evade the host immune system by residing latently within nervous system cells for extended periods of time, but can reactivate to become active infections during periods of stress and immunosuppression. For example, an initial VZV infection may result in a case of childhood chickenpox, followed by a long latency period. The virus can reactivate decades later, causing episodes of herpes zoster in adulthood. EBV enters latency in immune system B cells and possibly epithelial cells; it can reactivate years later to produce B-cell lymphoma.

    Exercise \(\PageIndex{5}\)

    Explain the difference between latent disease and chronic disease.

    Main Concepts and Summary

    • on ainfection, a microorganism enters a host and begins to multiply. Some infections causeillness, which is any deviation from normal host function or structure.
    • signalsof a disease are objective and measured.Symptomsof a disease are subjective and are reported by the patient.
    • Diseases can benon-infectious(due to genetics and environment) orinfectious(due to pathogens). Some infectious diseases arecommunicable(communicable between individuals) ortransmissible(easily transmissible between individuals); others areNOT communicable, but can be contracted by contact with environmental or animal reservoirs (zoonoses)
    • hospital illnessesare contracted in hospital environments, whileiatrogenic diseaseare the direct result of a medical procedure
    • Oneacute illnessis short-lived, whilechronic diseaselasts for months or years.latent diseasesthey last for years, but are distinguished from chronic diseases by the lack of active replication during long periods of dormancy.
    • Periods of illness include theincubation period, oprodromal period, operiod of illness, operiod of decline, it's atperiod of convalescence. These periods are marked by changes in the number of infectious agents and the severity of signs and symptoms.


    1. 1F. Savino et al. “Assessment of pain in children undergoing venipuncture: the Wong-Baker faces scale versus skin conductance fluctuations.”PeerJ1 (2013): e37;https://peerj.com/articles/37/
    2. 2J.G. Kusters et al. Pathogenese daHelicobacter pyloriInfection.Clinical Microbiology Reviews19 no. 3 (2006):449–490.
    3. 3N.R. Salama et al. “Life in the human stomach: bacterial pathogen persistence strategiesHelicobacter pylori.”Nature Reviews Microbiologia11 (2013): 385–399.
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