# Epidemiology --- **Epidemiology** is the study of [[nursing|health]] and [[disease]] patterns in populations. It's the broader view of how [[pathogen|pathogens]] or [[toxin|toxins]] move though and effect whole populations, and as such individuals are only looked at collectively. They search for causes of the disease, identify who is at risk, and determine how to control the spread or stop it alltogether. ## Terminology **Incidence:** The number of *new cases* of a disease in a population at a specific point of time of over a specific time. Can be described as "incidence rate" or "incidence proportion" **Prevalence:** The *total number* of disease in a population at a specific time, or over a specific time period. Can be described as "point prevalence" or "period prevalence." **Morbidity:** The number of *new cases* of disease in a population. **Mortality:** The number of people who *die* from a disease within a population. **Endemic:** A disease that is always present within a population; usually at low levels, like a cold virus, or malaria **Epidemic:** An occurrence of an unusually high number of disease cases; for example when the flu spreads during winter months. **Pandemic:** A worldwide epidemic of a disease; for example [[COVID-19]] ## Infectious Dose 50 **Infectious dose 50**, or ID$_{50}$, is the dosage of a pathogen required to make 50% of a test population sick. The smaller the number, the more [[pathogen#Virulence|virulence]] the pathogen is. For example, if you take 10 mice, and inject 3ml of the pathogen into all of them, and 5 get sick, the ID50 for that pathogen is 3ml. ## Koch's Postulates: Identifying The Cause of a Disease **Koch's postulates** are a set of criteria that are used to determine of a particular pathogen is the cause of a particular disease. They are as follows: 1. The microbe must be present in every case of the disease. 2. The microbe must be isolated and grown in pure culture from disease hosts. 3. The cultured microbe must cause the same disease when introduced to a healthy susceptible organism. 4. The microbe must be reisolated from that organism and be identified as the same one. Looking especially at postulate 4 we can see it is a thorough set of criteria. In modern times it has been replaced by more sophisticated measures. ### Molecular Koch's Postulates **Molecular Koch's postulates** are similar to Koch's regular postulates, but they are used to identify a particular [[virulence|virulence factor]] of a disease. The criteria are as follows: 1. The molecule must be present in all strains that cause disease. 2. Mutating the gene for that molecule must decrease the [[virulence]] of the microbe. 3. Restoring the gene must restore the microbe's virulence. Similar to Koch's postulates, molecular Koch's postulates are very thorough. ## Transmission of Disease There are many ways that a disease can be transmitted. The common portals of entry for a pathogen are: - [[eyes]] - [[nose]] - [[gastrointestinal tract]] - [[respiratory tracts]] - [[genitourinary tract]] - broken [[skin]] ### Disease Reservoirs **Disease reservoirs** are the natural habitat of a [[pathogen]] *and* a potential continuous source of disease. Simply being a place where the pathogen *is* isn't enough for it to be considered a reservoir, the pathogen needs to be able to survive and reproduce there. There are three types of disease reservoirs, humans, non-human animals, and the environment. **Humans** themselves can be reservoirs, but it's important to understand that the human may or may not show signs or symptoms of the disease. For example, humans are a reservoir of [[HIV]]. If an **animal** is a reservoir it is called a **zoonose**—or a disease that is mostly found in animals, but can be transmitted to people. For example, [[rabies]] is a zoonose. The **environment** can be a reservoir as well. Some pathogens live in the soil or water, and can be transmitted to humans. ### Vertical Transmission **Vertical transmission** is the transmission of a disease from mother to child, specifically when the baby is in utero, during childbirth, or during breastfeeding. If the disease is transmitted from mother to child, but it's done in any other way, this counts as [[#horizontal transmission]]. ### Horizontal Transmission **Horizontal transmission** is the transmission of disease in any way that is not [[#vertical transmission]]. - *direct contact* - *indirect contact*—through a *fomite*, or an inanimate object that can transfer disease, like a needle or doorknob. - *droplet*—transmission between two people less than 1 meter apart - *airborne*—transmission between two people more than 1 meter apart - *cross-contamination*—of food or water - *vector transmission*—when the pathogen lives in an insect, like in the plague, malaria, or lime disease. ## Risk Factors of Disease A **risk factor** of a [[disease]] is something that influences how likely it is that someone will get that disease. The four general categories for risk factors are: ### Herd Immunity **Herd immunity** is when a large portion of a population becomes [[immunity|immune]] to a [[disease]] (either from previously contracting it and fighting it off, or through a [[vaccine]]), making the spread of the disease unlikely. Below is a representation of three populations at varying levels of immunity. Note that there are only three infected hexes in each population, but the degree to which the population is vaccinated varies. ![[herd immunity before.png]] Now see how the infection has run it's course. In the 5% and 30% vaccinated population, the infection runs unchecked. In the 75% population however, the vast majority of the population reamins unaffected—even those who remained unvaccinated. ![[herd immunity after.png]] ### General Health and Living Conditions The types of living conditions that might increase the chance of getting the disease are malnutrition or overcrowding. ### Age The very young do not have a fully developed immune system, and the old have a weakened immune system, as a consequence of aging. ### Genetic Background Some people just have a natural advantage or disadvantage against certain pathogens. For example, [[HIV]] needs to bind to two different molecules to gain entry to our cells: CD4 and a particular type of chemokine receptor. Some people do not have that chemokine receptor, and are thus immune to HIV. ## Identifying Risk Factors Epidemiologists can identify risk factors of a disease by performing either a retrospective study (looking back to see if there is a pattern), or a prospective study (following a group with the risk factor and a control to see if one has an increased occurrence of the disease). ___