The R0 value is how epidemiologists quantify disease contagiousness

Understanding R0, a measure of how contagious a pathogen is

The R0 value is a basic measure of how many people an infected person will transmit a pathogen to, on average. If R0 is less than 1, transmission dies out; if greater than 1, an outbreak is likely. Measles has one of the highest known R0 values at almost 18, meaning that after four transmission steps, more than 100,000 people could be infected.

Measles is one of the most contagious preventable diseases

The virus’s ability to persist in the air for up to two hours means an infected person could enter a room, leave, and two hours later infect someone else entering the room. Its location in the body’s upper respiratory tract also makes it easy for viral particles to be sneezed or coughed into the air.

The measles vaccine has saved an estimated 94 million lives since its introduction

Measles killed an estimated 2.6 million people globally in 1980, after the development of a vaccine but before its widespread dissemination. By 2014, that figure had dropped to an estimated 73,000.

Rabies kills more than 50,000 worldwide each year

Rabies transmission to humans typically comes from an infected animal, and death was very common before recent protocols were developed (post-exposure prophylaxis). The virus is neurotropic, which means it has an affinity for nerve tissue, and moves quickly from broadly infecting the body to destroying the brain.

Receptors on the cell surface are key to whether a virus is successful

Viral receptors are key regulators of host range, tissue tropism, and viral pathogenesis—basically, the door locks on cells allowing viruses to invade, if they have the right key. How this process works at the microscopic level determines whether a virus is able to trigger a widespread illness in the host.

Herd immunity works by slowing down viral transmission until it dies out

Herd immunity refers to a population whose vaccination rates against a given virus are high enough that transmission dies out before the bug can find the next unvaccinated person to infect. Rates needed to achieve this depend on how contagious a virus is. This video gives a quick visual explanation of how it works.

The Reynolds number is the ratio of viscous and inertial forces in fluids

It can approximate fluid behavior in simple systems by non-dimensionalizing—removing the units within—the Navier-Stokes equations. A value close to zero indicates that the fluid will exhibit laminar flow, while a large number suggests that turbulence will occur.

Leprosy bacteria replicate extremely slowly—only once every two weeks

Mycobacterium leprae has a long incubation period and a very slow replication rate, taking roughly 12 to 14 days to make a copy of itself and divide. In contrast, bacteria like E. coli can replicate in about 20 minutes in ideal conditions. Someone can be infected for years without experiencing any symptoms. This also makes the disease difficult to treat. This video describes the knowns—and unknowns—about this infectious bacterium.

What is the measles, mumps, and rubella vaccine?

Federal guidance currently recommends a two-shot schedule of the MMR vaccine for children in most cases, once at 12 to 15 months and once at 4 to 6 years. (A September 2025 update only called for the elimination of a combined vaccine that included protection against varicella, the MMRV shot.)

A heatmap showing the drop in infectious diseases post-vaccine deployment

The number of infected people, measured over 70-some years and across all 50 states and the District of Columbia, declined significantly after vaccines were introduced.

What are zoonotic diseases?

Interspecies interactions are a breeding ground for disease transmission. This basic background information on zoonotic disease from the Centers for Disease Control and Prevention (CDC) chronicles the different ways humans can get sick, identifies what groups are particularly vulnerable, and offers links to other resources.

An Ebola vaccine was developed during the 2018 west African Ebola outbreak

A study found that Merck's rVSV-ZEBOV Ebola vaccine (called Ervebo) was 84% effective against infection during the 2018-2020 Ebola outbreak in the DRC. This effect was observed for both sexes and among children and adults. The vaccine is approved to help provide protection against one specific species of the Ebola virus. The 2026 Ebola outbreak is with a different species of the virus—not the one that the vaccine is approved to treat.

Watch how viruses like Ebola can spill over from animals to humans—and vice-versa.

What is metabolic rate?

Read this article to better understand the concept of metabolic rate, which is defined as the amount of energy expended by an organism over the course of a day. Basic metabolic rate (or BMR) is this metabolic rate at rest, during which the body only...

Resistant infections are no longer isolated and can spread worldwide

What does the rise of antibiotic resistance mean in practical terms? Maryn McKenna is a journalist at the forefront of covering antibiotic resistance since 2007. In this TED talk, McKenna lays out the very real consequences of antibiotic resistance—and what needs to be done to address the problem.

Calculate your estimated yearly dose of ionizing radiation

This calculator from the EPA provides a value in millirems—the US unit for effective dose—based on a variety of exposure parameters, including the state a user lives in and travel-related sources of ionizing radiation, such as passing through screening machines.

Estimates say 2.5 billion total T. rexes lived on Earth

Roughly 68 million years ago, the Tyrannosaurus rex was king of the North American ecosystem. Researchers used the dinosaur's size, metabolic rate, age of sexual maturity, and other factors to determine 20,000 individuals likely were alive at any given time, more than 120,000 20-year generations. This piece breaks down how they arrived at those numbers.

The history of germ theory, which explains that illnesses are caused by microorganisms

For centuries, the miasma theory, which held that illnesses were caused by clouds of "bad air" produced by decaying organic matter, was the accepted explanation for the spread of disease. The development of microscopes and the subsequent technology to isolate bacteria and test disease spread in mice greatly contributed to contagionists overturning miasmatists.

Visualizing the genetics of SARS-CoV-2

The COVID-19 pandemic, caused by the SARS-CoV-2 virus, began with an outbreak in Wuhan, China in December 2019, and rapidly spread worldwide in early 2020. On March 11, 2020, the World Health Organization declared COVID-19 a global pandemic, leading to widespread lockdowns, travel restrictions, and economic disruptions.

Rheumatoid arthritis affects about 1% of the population

Rheumatoid arthritis is an autoimmune condition leading to breakdown and inflammation of multiple joints throughout the body. This academic article provides an updated deep dive into our current cellular understanding of rheumatoid arthritis, how it is diagnosed, and the variety of medications available for treating this painful condition.

What is the Responsibility to Protect?

This website provides an in-depth overview of the Responsibility to Protect. An important global principle, R2P seeks to safeguard populations from genocide, war crimes, crimes against humanity and ethnic cleansing. The website’s ‘What is...

1 in 31 kids in the US has autism

That figure comes from an analysis published by the US Centers for Disease Control and Prevention based on data gathered on eight-year-olds in 2022. Its figures mark a significant increase from 2000, when the estimated autism figure was one in 150, but improvements in diagnosis and changes to the disease criteria that broadened the diagnostic criteria are widely believed to be driving that increase.

There are between 10 and 100 trillion bacteria in the gut

The composition of our microbiome is essential to overall health. Anything that disrupts the balance of the trillions of bacteria may be harmful—even treatments meant to fight infection, like antibiotics.

Genetics may account for nearly 67% of mosquito attractiveness

In a mosquito attraction study published in 2015, identical twins appeared equally attractive to mosquitoes in lab tests, whereas some nonidentical twins were far less likely to be bitten than their siblings. Other work related showed that body odor is also more similar between identical twins.

See the differences between chickenpox and hand, foot, and mouth

This helpful explainer breaks down the differences between these two contagious, viral diseases, which both present with fevers and sores. It includes photos of the sores that are characteristic of both diseases, showing how they have appeared across different skin tones to better inform people looking for information about their own loved one's symptoms.

Network effects are the economic term for a good or service's popularity

When goods or services become especially popular, firms that provide that good or service become dominant in the market—which can ultimately lead to a monopoly.

Between 10 million and 50 million people in the US develop rashes from poison ivy, poison sumac, and poison oak each year.

Radioactive elements can introduce software errors because the ionizing radiation they produce can detach electrons from atoms, and computer code relies on the distribution of electrons.

Visualize how CRISPR-Cas9—a type of immune system used by single-celled organisms to attack invasive viruses—targets and makes precise changes to a gene's DNA.

Hookworm infection, though risky, may help fight certain autoimmune conditions by tamping down the immune system's inflammatory response.

Coral polyps reproduce via broadcast spawning, where entire reefs simultaneously release reproductive cells for about one hour, once a year.

Indefinite cloning of some endangered species may not sustain their longevity because cloning success rates decrease as genetic mutations accumulate.

The hormonal disorder PCOS was renamed PMOS in 2026, a move designed to improve care and research by shifting the focus to the hormonal causes of this condition that affects 1 in 8 women.

Bone marrow (soft tissue found inside certain bones) generates roughly 500 billion new blood cells each day

The American cockroach is actually from Africa, an apparent hitchhiker aboard ships carrying enslaved people, with evidence suggesting the bug may have arrived in the New World as early as the 1600s.

Use this calculator to find the dollar value of your mental load, from meal planning to coordinating after-school care for your children.

The deadliest aspect of volcanic eruptions is often not lava, but pyroclastic flows—avalanches of ash, gas, and rock that can move more than half the speed of sound.

Fireworks-related ER visits are on the rise—in 2024, they increased by 52% as compared to the prior year.

Louisiana's 'Cancer Alley' is infamous for having some of the highest industrial pollution, toxic air emissions, and cancer risks in the US.

Chaos theory—informally known as the butterfly effect—is the reason why weather forecasts are presented with probabilities rather than certainties.

Some coin denominations have ridges etched on their sides, partly to prevent counterfeiting, in a practice called 'reeding.'

The Russia-Ukraine conflict is the largest ground war in Europe since World War II in casualties, forces mobilized, and displacement of civilians.

Neutron stars, the remains of some massive stars after they explode in a supernova, are the densest directly observable objects in the universe.

The Nasdaq's 'Magnificent 7' companies are responsible for roughly 40% of the Nasdaq 100 index's gains, while the remaining 93 companies account for the other 60%.

Lesser-known phobias include turophobia, or the fear of cheese, and nomophobia, the fear of being without your cell phone.

All rainbows are 'raincircles' caused by water droplets separating white sunlight into a spectrum of colors, but Earth's surface blocks the bottom half of the circle from our line of sight.

44% of Americans say they pray at least once a day—discover how your state's religiosity stacks up against the rest of the country.

The first steps to rebuilding your credit score include getting to know exactly what's on your credit report and figuring out how far you are from the '700 club,' or 'good score' range.

The distributor behind 'Rocky Horror' pulled the film from its scheduled national release, believing it would be a failure, though it eventually amassed a dedicated fanbase through midnight showings, a trend that continues over 50 years later.

The term 'algorithm' originated from the Latinized name of al-Khwārizmī, a 9th-century mathematician whose works introduced Arabic numerals and the decimal-based numeral system to Europe and whose treatise 'Al-Jabr' gave algebra its name.