1.1-What is a disease?
A disease is any condition in which the normal structure or functions of the body are damaged or impaired. Physical injuries or disabilities are not classified as disease, but there can be several causes for disease, including infection by a pathogen, genetics (as in many cancers or deficiencies), noninfectious 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 noninfectious causes.
Signs and Symptoms of Disease
An infection is the successful colonization of a host by a microorganism. Infections can lead to disease, which causes signs and symptoms resulting in a deviation from the normal structure or functioning of the host. Microorganisms that can cause disease are known as pathogens.
The signs of disease are objective and measurable, and can be directly observed by a clinician. Vital signs, which are used to measure the body’s basic functions, include body temperature (normally 37 °C [98.6 °F]), heart rate (normally 60–100 beats per minute), breathing rate (normally 12–18 breaths per minute), and blood pressure (normally between 90/60 and 120/80 mm Hg). Changes in any of the body’s vital signs may be indicative of disease. For example, having a fever (a body temperature significantly higher than 37 °C or 98.6 °F) is a sign of disease because it can be measured.
In addition to changes in vital signs, other observable conditions may be considered signs of disease. For example, the presence of antibodies in a patient’s serum (the liquid portion of blood that lacks clotting factors) can be observed and measured through 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 an active disease. Antibodies can remain in the body long after an infection has resolved; also, they may develop in response to a pathogen that is in the body but not currently causing disease.
Unlike signs, symptoms of disease are subjective. Symptoms are felt or experienced by the patient, but they cannot be clinically confirmed or objectively measured. Examples of symptoms include nausea, loss of appetite, and pain. Such symptoms are important to consider when diagnosing disease, but they are subject to memory bias and are difficult to measure precisely. Some clinicians attempt 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 0–10. An alternative method of quantifying pain is measuring skin conductance fluctuations. These fluctuations reflect sweating due to skin sympathetic nerve activity resulting from the stressor of pain.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. Table 1.1 lists some of the prefixes and suffixes commonly used in naming syndromes.
| Nomenclature of Symptoms | ||
|---|---|---|
| Affix | Meaning | Example |
| cyto- | cell | cytopenia: reduction in the number of blood cells |
| hepat- | of the liver | hepatitis: inflammation of the liver |
| -pathy | disease | neuropathy: a disease affecting nerves |
| -emia | of the blood | bacteremia: presence of bacteria in blood |
| -itis | inflammation | colitis: inflammation of the colon |
| -lysis | destruction | hemolysis: destruction of red blood cells |
| -oma | tumor | lymphoma: cancer of the lymphatic system |
| -osis | diseased or abnormal condition | leukocytosis: abnormally high number of white blood cells |
| -derma | of the skin | keratoderma: a thickening of the skin |
Clinicians must rely on signs and on asking questions about symptoms, medical history, and the patient’s recent activities to identify a particular disease and the 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 presenting with symptoms of diarrhea may have been infected by one of a wide variety of pathogenic microorganisms. Bacterial pathogens associated with diarrheal disease include Vibrio cholerae, Listeria monocytogenes, Campylobacter jejuni, and enteropathogenic Escherichia coli (EPEC). Viral pathogens associated with diarrheal disease include norovirus and rotavirus. Parasitic pathogens associated with diarrhea include Giardia lamblia and Cryptosporidium parvum. Likewise, fever is indicative of many types of infection, from the common cold to the deadly Ebola hemorrhagic fever.
Finally, some diseases may be asymptomatic or subclinical, meaning they do not present any noticeable signs or symptoms. For example, most individual infected with herpes simplex virus remain asymptomatic and are unaware that they have been infected.
