Classifications, Types, and Terminology of Disease

1. Introduction and Characteristics of Infectious Disease

In this lesson, you will learn how infectious disease differs from other types of disease and from other conditions. The general term disease is defined as any condition in which the normal structure and/or functions of the body are damaged or impaired in some way. For example, someone with a severe respiratory infection (a type of disease) may experience damage to their lung tissue and difficulty obtaining enough oxygen to support normal body functions.

Infectious diseases result from the direct effects of some sort of pathogen. In contrast, there are many diseases and other conditions that are not infectious diseases. The table below describes types of noninfectious diseases and their definitions. Other conditions that affect the ability to function include physical injuries and disabilities, which are not considered diseases.

The image below shows examples of infectious versus noninfectious disease in blood smears. In part (a), there is a highlighted protist (Plasmodium falciparum) surrounded by red blood cells (erythrocytes). P. falciparum is one of the pathogens that causes an important infectious disease, malaria. In part (b), there is a blood smear showing red blood cells with no evidence of protists. However, some of the cells have a sickle (crescent) shape. This type of cell is characteristic of the noninfectious disease sickle cell anemia, which is an inherited genetic disorder.

When a patient presents to a medical professional (i.e., shows up for evaluation) asking questions about whether they are sick and in need of treatment, the medical professional needs to consider whichever of these possible causes may be responsible. Sometimes infectious and noninfectious diseases or other conditions can appear similar. However, this course will primarily focus on infectious diseases and how they are caused by microbes with an important exception: in the lesson on the immune system, you will learn how an inappropriate immune response can cause disease.

To study disease, it is often helpful to learn basic word parts that are commonly used in medical terms. These word parts often have a Latin or Greek origin. The table below gives definitions and examples of some common word parts. You may have heard some of these word parts before.

EXAMPLE

Arthritis contains the word parts “arthr/o” (joint) and “itis” (inflammation). Arthritis refers to inflammation of a joint.

terms to know

Disease

Any condition in which the normal structure and/or functions of the body are damaged or impaired in some way.

Infectious Disease

A disease resulting from the direct effects of a pathogen.

2. Signs and Symptoms of Disease

An infection is the successful colonization of a host by a microorganism. Infections can cause disease and this may cause the host to experience changes related to the infection. These changes are grouped into two categories: signs and symptoms.

The signs of a disease are objective and measurable, meaning that they can be observed by a clinician. For example, medical visits often begin with a health care professional taking vital signs to note any changes that may be signs of disease. Vital signs include body temperature, heart rate, breathing rate, and blood pressure. If someone has an unusually high body temperature (i.e., a fever), then that is a sign of disease. Other changes in vital signs can also signify the presence of an infection. Other examples of signs include measurements from blood analysis (e.g., white blood cell counts).

The symptoms of a disease are subjective. They are experienced by the individual who is sick and can be described but cannot be directly, objectively measured by a clinician even though the clinician can attempt to assess them (e.g., by asking a patient to rate their level of pain on a pain scale). Examples of symptoms include nausea, loss of appetite, and pain. It is important to consider symptoms when diagnosing disease. However, sometimes significant symptoms may be present without objective signs of disease, but symptoms can be more difficult to assess. For example, different people may rank pain differently on a pain scale.

A specific group of signs and symptoms characteristic of a particular disease is called a syndrome. Syndromes are often named using a nomenclature based on signs and symptoms using the word parts in the table above. However, some signs and symptoms are characteristic of multiple conditions and it is not always easy to identify the underlying cause of relatively nonspecific patient concerns such as abdominal pain. Many types of infections cause fevers. Therefore, doctors need to combine information on signs and symptoms with the patient's responses to questions about medical history, recent activities, and potential recent exposures (such as contact with an individual who has a particular condition) to determine appropriate tests and follow-up.

In some cases, diseases are asymptomatic (meaning that they do not produce noticeable signs or symptoms) or subclinical (meaning that any signs and symptoms are relatively mild and therefore relatively difficult to observe and diagnose). People with asymptomatic or subclinical infections may not realize that they are sick.

try it

You go to the doctor complaining that your throat is very painful. The doctor looks, but your throat looks healthy and is not inflamed.

Is your throat pain an example of a sign or a symptom?

It is an example of a symptom because you can feel and report it, but the doctor cannot see and measure it.

try it

You go to the doctor for a standard exam prior to competing as a swimmer. The doctor notes that your blood pressure is 152/94 and expresses concern, asking if it has been high previously and opening a discussion about what to do next.

Is your high blood pressure an example of a sign or a symptom?

It is an example of a sign because it is measured using a medical instrument (technically, a sphygmomanometer).

terms to know

Infection

The successful colonization of a host by a microorganism.

Sign (of Disease)

A characteristic of a disease that is objective and measurable, meaning that it can be observed by a clinician.

Symptom (of Disease)

A characteristic of a disease that is subjective and cannot be directly, objectively measured by a clinician.

Syndrome

A specific group of signs and symptoms characteristic of a particular disease.

Asymptomatic Infection

An infection that does not produce noticeable signs or symptoms.

Subclinical Infection

An infection that produces relatively mild signs and symptoms that are therefore relatively difficult to observe and diagnose.

3. Classifications of Disease

Diseases can be classified in a variety of ways. The World Health Organization’s (WHO) International Classification of Diseases (ICD) is used in clinical fields to classify diseases and ICD codes are used to designate specific conditions for purposes such as record-taking and billing.

Infectious diseases can be classified into three categories based on the way they spread. Communicable diseases can spread from one person to another by direct or indirect mechanisms. Some communicable diseases are also considered contagious diseases, meaning that they can easily spread from one person to another. Noncommunicable diseases, such as tetanus, do not spread from person to person and are acquired in other ways (e.g., through the entry of the pathogen that causes tetanus, Clostridium tetani, into a wound).

Contagious diseases vary in how easily they are transmitted. Measles is one of the most contagious diseases known and each infected individual can easily spread the disease to multiple others. Each time the infected individual speaks or coughs, viral particles are expelled into the air that can infect others. In contrast, gonorrhea is less contagious because it requires close intimate contact (such as sexual contact) to spread the pathogen.

When diseases are contracted as the result of a medical procedure, they are termed iatrogenic diseases. This can occur when a person undergoes a procedure and a pathogen is somehow introduced into a wound or surgical site. This can also result from catheterization.

Nosocomial diseases (also called health-care-acquired infections, hospital-acquired infections, or HAIs) are diseases acquired in hospitals. These can be highly problematic. People go to hospitals when they are sick, meaning that many pathogens are present and strict protocols (such as frequent handwashing and cleaning of surfaces) are needed to reduce the risk of spread. Secondly, many hospital patients have weakened immune systems that increase their risk of contracting an infection. Finally, although antibiotics are an important treatment tool in hospitals, their use increases the risk that antibiotic-resistant bacteria will be present. Unfortunately, resistant pathogens are common in hospital settings for this reason.

Finally, infections that can be spread from animals to humans are called zoonotic diseases or zoonoses. The term can be restricted to vertebrate animals, but is sometimes used more broadly.

EXAMPLE

A well-known zoonotic disease is rabies, which is caused by the rabies virus and spread through animal bites or contact with infected saliva.

terms to know

Communicable Disease

A disease that can be spread from one person to another by direct or indirect mechanisms.

Contagious Disease

A disease that can be easily spread from one person to another.

Noncommunicable Disease

A disease that does not spread from person to person but instead is acquired in other ways.

Iatrogenic Disease

A disease contracted as the result of a medical procedure.

Nosocomial Disease

A disease acquired in a hospital or similar setting.

Zoonotic Disease (Zoonosis, plural Zoonoses)

Infections that can be spread from animals to humans.

4. Periods of Disease

When someone is exposed to a pathogen that causes infectious disease, it begins a series of five periods of disease (also called stages or phases of disease). Throughout these phases, the number of pathogen particles increases at first. As long as the patient survives, the number of pathogen particles then decreases. The graph below shows how the number of pathogen particles (red) and severity of symptoms (blue) change over time as a particular infection moves through the five periods of disease. The lengths of the periods and details of the pattern vary with the type of infection and differences between acute diseases (which occur over a relatively short time) and chronic diseases (which last for a longer time) will be discussed later in this lesson.

step by step

Step 1: After a pathogen enters a host, the incubation period begins. The pathogen begins to multiply during this period, but the pathogen is in low abundance, and therefore signs and symptoms are not yet present. The incubation period generally lasts for a day or two (or slightly more) for acute diseases but may last months to years for chronic diseases. The length of the incubation period is affected by many factors including:

• the type of pathogen
• the number of pathogen particles introduced (i.e., the infectious dose)
• the strength of the host immune system
• the site of infection
• the type of infection

The patient is unaware of the disease during this period.

Step 2: Next, the prodromal period begins. During this phase, the pathogen continues to multiply, and the host begins to experience the general signs and symptoms of disease. These signs and symptoms are often caused by the immune response and can include fever, pain, soreness, swelling, or inflammation. It is often difficult to diagnose disease at this point because the symptoms are relatively general.

Step 3: After the prodromal period, the period of illness begins. At this stage, the signs and symptoms of the disease are most severe and clear. This means that the disease may be easier to diagnose.

Step 4: The period of decline begins as the number of pathogen particles begins to decrease. The signs and symptoms also decrease during this period. However, individuals may be weakened from their illness and have increased susceptibility to developing other infections. The primary infection (meaning the infection that caused the original illness) may have caused damage that allows another pathogen to begin to multiply and cause a secondary infection. The development of a secondary infection can lead to a sudden worsening of symptoms (e.g., the sudden return of a fever) that merits evaluation by a medical professional.

Step 5: Finally, the period of convalescence is the stage during which the patient returns to normal functioning. However, it is possible that there may be some residual damage, or even permanent damage, from the infection.

Diseases and pathogens differ in when they are most contagious but can be contagious at any point during the five periods of disease. Knowing when a disease is contagious is important in helping to control its spread. Pathogens that are contagious during the incubation period can be especially difficult to control as people are active while not realizing that they are harboring a pathogen. However, other pathogens (such as some diarrheal diseases) may continue to be shed in feces for a long time and this leads to an extended period of contagiousness.

terms to know

Acute Disease

A disease that occurs over a relatively short time.

Chronic Disease

A disease that occurs over a relatively long time.

Incubation Period

The period after a pathogen enters a host but before signs and symptoms develop.

Prodromal Period

The period during which the pathogen is multiplying in the host and the host begins to experience signs and symptoms, but the signs and symptoms may be nonspecific, and the disease is often difficult to diagnose.

Period of Illness

The period during which the signs and symptoms of the disease are most severe and clear, meaning that the disease is often easiest to diagnose.

Period of Decline

The period during which the number of pathogen particles and the signs and symptoms of the disease begin to decrease.

Period of Convalescence

The period during which the patient returns to normal functioning, although there may be remaining long-term damage from the infection.

5. Acute Versus Chronic Diseases

As discussed earlier in the lesson, diseases can be acute or chronic. They can also be latent, meaning that the causal pathogen goes dormant for extended periods of time with no active replication.

Acute diseases involve pathologic changes during a relatively short time (usually ranging from hours to a few weeks). The onset of the disease is relatively rapid and the disease is not long lasting.

EXAMPLE

The period of decline for influenza begins after approximately 1 week.

Chronic diseases involve the appearance of pathologic changes over longer time spans ranging from months to a lifespan. The pathogen continues to reproduce over this time.

EXAMPLE

Helicobacter pylori bacteria can live in the stomach and cause recurring infections potentially associated with complications such as ulcers and stomach cancer (Ho et al., 2022). Hepatitis B virus can cause a chronic liver infection.

In contrast with chronic diseases, latent diseases occur when pathogens have periods of dormancy.

EXAMPLE

Chickenpox (caused by varicella-zoster virus), herpes (caused by herpes simplex viruses 1 and 2), and mononucleosis (caused by Epstein–Barr virus) enter a latent form but can reactivate to become active infections during times of stress and immunosuppression. Varicella-zoster virus can cause chickenpox in childhood and then reactivate in adulthood to cause shingles. Epstein–Barr virus can go into latency in B cells of the immune system and possibly epithelial cells, then reactivate later to cause B-cell lymphoma.

term to know

Latent Disease

A disease in which the causal pathogen goes dormant for extended periods of time with no active replication.

REFERENCES

Ho, J. J. C., Argueta, E. A., & Moss, S. F. (2022). Helicobacter pylori treatment regimens: A US perspective. Gastroenterology & hepatology, 18(6), 313–319. europepmc.org/article/med/36398140