It will definitely be necessary for you to find more information about the topics covered in this area. We have just provided some examples and guidelines to help you know what to look for.
These are the body systems for which you will need to identify aspects of disease. The systems have all been described in a previous section of this guide.
Integumentary Musculoskeletal Nervous Cardiovascular, hematopoietic, and lymphatic Respiratory Digestive Urinary Reproductive Endocrine Sensory
You will need to explore the following, as they relate to the body systems listed under the previous topic. You may find it helpful to make a chart from which to review.
Disease processes: This is the less technical term for “pathophysiology” or the course a disease takes as it progresses through the body. Focus on the functional changes and disturbances that ultimately result in observable signs or subjective symptoms. This could also be referred to as the “why” and the “how” of a disease.
Diagnosis and treatment modalities: Diagnosis is the means of identifying an illness or injury. This may include physical examination and imaging or require more invasive tests such as blood work. Note that a “differential diagnosis” is the process by which two or more diseases with similar presentations are distinguished. The treatment modality will depend on the disease; some can be treated with medications, some require surgery, others can be alleviated with lifestyle modification. Not all treatments are curative and not all diseases are curable so the aim of therapy may simply be to alleviate unpleasant symptoms such as pain.
Incidence and prevalence: These terms are very commonly confused. Incidence refers to the number of new cases of a disease in a population over time (typically one year). Prevalence is the number of people who have a disease right now (or at an isolated point in time). Prevalence is typically indicated as a percentage of the total population.
Risk factors: A risk factor is any physical trait, lifestyle practice, or other characteristic of a patient that increases their likelihood of contracting a disease or suffering an injury. Risk factors may be modifiable (such as poor diet and smoking) or non-modifiable (such as age, race, and gender).
Conditions or states of health: A medical condition or disorder is sometimes distinguished from a disease as an abnormal state of health that impairs a patient’s activity or well-being but may not be related to a known disease process. More generally, “condition” may also refer to a patient’s overall quality of life or state of being. It is often used in discussions about a patient’s potential outcome (also known as a prognosis).
Health-related syndromes: Syndromes are collections of subjective symptoms and objective signs associated with a disease process or used to diagnose a disease. For example, trisomy 21 is a disorder that results in the symptoms known as Down syndrome. Cushing syndrome is the set of signs often looked for in suspicion of hypercortisolism. Some syndromes have no known cause or only a poorly understood one (such as chronic pain syndrome or restless leg syndrome).
Complications: Complications are unexpected, often harmful, events that occur secondary to a disease or sometimes during treatment. They may also be called or result from “adverse effects.” It is important to distinguish these problems from side effects, which are signs/symptoms experienced during disease or therapy that may be unwanted but are not unanticipated.
Accompanying diseases: This is the less technical term for “comorbidities” or secondary disorders that occur at the same time as the primary disease. These may be related or seemingly unrelated; they often include behavioral or mental health concerns. Accompanying diseases are often associated with worse patient outcomes.
An epidemic is an outbreak of contagious disease that infects a large number of people in a particular area. The United States has experienced a fair number of epidemics with some of the deadliest being smallpox, polio, and Spanish flu. However, the infection rate, not the death rate, determines whether an occurrence is deemed an epidemic. Recent outbreaks of pertussis, measles, and mumps have quickly spread in unvaccinated regions of the U.S. Some outbreaks are also considered epidemics because they occur in an area where the disease is not typically found (such as eruptions of yellow fever, which is a tropical disease, in the northern U.S.).
When an epidemic has reached global proportions it is referred to as a pandemic, though there are not specific criteria for how many countries a disease must reach to earn this status. Malaria, tuberculosis, and HIV are all non-eradicated diseases generally accepted as pandemic.
Questions in this area will cover body organization at the cell level, from basic cell structure to cell reactions to disease.
The cell is the smallest structural unit in the body as well as the smallest unit of all life. However, cells are composed of even smaller structures called organelles (meaning “little organs”). Unlike a single cell, an isolated organelle cannot survive and replicate on its own.
Cell wall ― No animal cells (including human cells) have cell walls. Only plant cells, bacterial cells, and fungal cells are surrounded by this organelle. The cell wall is a protective layer of either cellulose or chitin (depending on the species) that encloses the cell membrane. This makes it the most external organelle when present. It is often rigid, as it is responsible for maintaining cell structure and providing defense, but it may be flexible and semi-permeable for the exchange of nutrients necessary to cell life.
Cell membrane ― All cells are bound by a cell membrane that holds all other substructures in place. In humans and animals, there are two polarized layers made up of lipids and proteins that adhere to one another to create a strong, flexible, semi-permeable sheet. In bacteria, there is only one layer in the plasma membrane. This organelle is essential to cell signaling and communication as well as separating the cell from its external environment. The cell membrane is also where diffusion of molecules and excretion of waste occurs.
Cytoplasm ― Cytoplasm is the intracellular material within which all organelles are suspended. Its specific composition is not fully understood, but it is divided between cytosol (a fluid-like gel) and cytoskeleton (a rigid network of filaments that help establish cell structure and provide pathways for intracellular travel).
Nucleus ― The nucleus is a membrane-bound organelle that contains the majority of a cell’s genetic material or DNA in the form of chromosomes. The nucleus is the site of cellular gene “expression,” which dictates the functions a cell will ultimately be able to perform. This organelle is also responsible for replicating DNA prior to cell division.
Nucleolus ― The nucleolus, contained within the nucleus, is where ribosomes are created and manufactured. It also has a role in detaining harmful proteins in response to cellular stress.
Mitochondria ― Mitochondria (singular: mitochondrion) are often called the “powerhouses of the cell” because these organelles produce most of the cell’s energy in the form of a molecule called ATP. This process is known as cellular respiration and relies on the presence of oxygen. Most human cells have numerous mitochondria, which also participate in various metabolic tasks depending on their specialized location. Mitochondria also contain their own genetic material, independent of the nucleus, that is particularly important to human reproduction.
Endoplasmic reticulum ― The endoplasmic reticulum is an extensive series of flattened tubes or sacs that are connected to the nuclear membrane and take up much of the cell volume. This organelle is either rough (meaning it is coated in ribosomes) or smooth depending on the cell’s designated function. Rough endoplasmic reticulum is responsible for protein synthesis while smooth endoplasmic reticulum builds lipids and detoxifies many natural compounds.
Ribosomes ― Ribosomes are the specific site of amino acid assembly to generate proteins (hence their attachment to the rough endoplasmic reticulum). However, ribosomes can also be freely suspended in the cytoplasm.
Golgi apparatus ― The Golgi apparatus (also known as the Golgi complex or body) is similar in appearance and proximity to the endoplasmic reticulum. This is because the Golgi complex sorts and distributes the molecules produced by the endoplasmic reticulum, which are transported via membrane-bound vesicles. The Golgi body is also where lysosomes are formed.
Lysosomes ― A lysosome is a type of vesicle containing enzymes used to break down or “digest” the molecules taken in by the cell as well as damaged portions of the cell itself.
Human cells are eukaryotic, meaning even though each cell has a specialized shape and function, they all contain a nucleus and membrane-bound organelles (two exceptions are red blood cells and platelets, which have no nuclei, ribosomes, or mitochondria). Some human cells, notably certain bone and muscle cells, have more than one nucleus.
Prokaryotic cells contain no nucleus or organelles. All bacteria are prokaryotes and are all also composed of a single cell. Fungi on the other hand are eukaryotic, which is one of the reasons fungal infections are so difficult to treat (fungal cells are more similar to ours). Protozoans, a type of infectious parasite, are also eukaryotes but are unicellular like bacteria. Viruses and prions are technically non-cellular and require host cells to “live” and replicate.
A pathogen is an infectious agent, often a microorganism, such as a protozoan or bacterium. However non-living microbes, like viruses and prions, are also considered pathogens. Pathogens are distinguished by their ability to cause disease in humans. Some more common pathogens are listed below by type.
Bacteria: In reality, very few bacteria are pathogenic. However, the strain Streptococcus pyogenes commonly causes pharyngitis while Streptococcus pneumoniae and Pseudomonas aeruginosa can both trigger pneumonia. Bacteria are also responsible for the diseases of diphtheria, botulism, salmonella, shigella, syphilis, tuberculosis, and tetanus. Many severe bowel diseases are the result of opportunistic bacterial infections (such as by Clostridium difficile). Although bacterial infections were once easily eliminated with the use of antibiotics, newer infections are becoming increasingly resistant to medication resulting in “superbugs” or “multidrug resistant” (MDR) strains.
Fungi: Serious fungal infections typically only occur in patients with weakened immune systems. Candida albicans is often responsible for thrush and most yeast infections. Aspergillosis is a fungal infection caused by the genus Aspergillus while histoplasmosis is the result of Histoplasma capsulatum. Meningitis and pneumonia can both also be caused by fungi; the primary culprits are Cryptococcus and Pneumocystis, respectively.
Parasites: Parasites include internal protozoa as well as worms. Some of the most common parasitic protozoa belong to the genus Plasmodium; many of these species are responsible for malaria. Members of the genus Giardia cause giardiasis while Toxoplasma gondii causes toxoplasmosis. Parasitic worms are more commonly referred to as either hookworms, tapeworms, or whipworms based on their specific morphology. Some parasites are also external mites that cause infection by biting the skin: these include scabies, lice, and ticks.
Viruses: Viruses are non-living, non-cellular structures that require a host cell to replicate so they are all pathogenic (although not necessarily always to humans). The most prevalent families of viruses are Herpesviridae (including herpes, varicella, Epstein-barr, and CMV), Picornaviridae (causing hepatitis A and polio), and Flaviviridae (responsible for hepatitis C, yellow fever, dengue fever, and West Nile). Hepatitis B, smallpox, measles, mumps, rubella, rabies, HIV, and, of course, influenza are all viral illnesses. Viral diseases may be prevented by vaccination but are technically incurable. Because viruses live within host cells, they are difficult to eradicate and can lie dormant/in remission for years without causing symptoms.
Prions: Prions, like viruses, are non-living and non-cellular. Unlike viruses, which contain genetic material, prions are made entirely of protein and their mechanism for disease is not fully understood. All known prions infect the nervous system and all are untreatable and fatal. The two most common diseases are “mad cow disease” (BSE) and Creutzfeldt-Jakob disease.
Note that many common diseases are not always produced by the same species or even the same type of pathogen (consider pneumonia and meningitis which can be bacterial, viral, or fungal). The resulting disease also depends on where in the body the pathogen reproduces. Most pathogens favor a particular environment, but are still infectious outside it (for example, tuberculosis prefers to live in the lungs, but can infect the spine as well).
Non-pathogens are similarly microscopic entities that are harmless to humans because they do not cause disease if they are encountered or ingested. Many non-pathogens live within or on our body and some even help to carry out necessary microscopic functions in our gut or skin (these are typically referred to as our “normal flora”). Remember that the vast majority of bacteria are non-pathogens.
Sometimes, however, the difference between pathogens and non-pathogens is not clear. A bacterium that naturally and harmlessly lives on our skin (such as Staphylococcus aureus) or in our bowel (such as Escherichia coli) may also cause an infection if a wound or injury allows it to enter a new region of the body, or if a new strain is introduced (such as by food poisoning).
An organism is any living being including humans, animals, plants, fungi, and bacteria. Organisms vary widely in complexity and appearance but can all be identified as either a single cell or a collection of cells.
Unicellular or other microscopic organisms are further classified as microorganisms. The most prevalent microorganisms are prokaryotic bacteria and similar unicellular creatures known as archaea. Non-bacterial organisms may also be microscopic and consist of protists (mold), certain animals (dust mites), some fungi (yeast), and plants (algae).
In the absence of cellular features, a structure is not considered a living organism, even if it behaves in a way that appears to mimic life. Because organisms can be isolated to a single cell, all organisms are able to grow, survive, and reproduce on their own.
On the other hand, viruses are infectious agents (smaller than bacteria) that can only replicate inside of a living cell because they consist only of genetic material (DNA or RNA) surrounded by a protein shell; they lack the organelles necessary for metabolism and replication. Prions (even smaller than viruses) also fall into this category because they are made entirely of protein, with no DNA or RNA. Viruses and prions only “survive” based on their transmission among organisms. Their existence outside of the body is not well known.