Herd immunity works by slowing down viral transmission until a disease dies out.

Some vaccines, like those for measles, rely on weakened versions of a virus. These weakened pathogens are introduced into the body, triggering an immune response and helping the immune system learn to protect itself against stronger versions of the virus. (This also explains why some vaccines make you feel under the weather.)

As viruses mutate and develop, vaccines have to adapt, leading to regular recommended injections like flu shots. Researchers are working to develop universal vaccines that aim to protect against multiple virus variants if not entire virus families. These vaccines work by creating broadly neutralizing antibodies (bnAbs) that target aspects of viruses that don't typically mutate and are integral to the virus' survival.

Before the technology behind vaccinations was discovered, some keen observers of medicine around the globe noticed how previous infections could impact future ones. Learn about the techniques early physicians used before they had vaccines.

The diseases with the greatest potential to spread widely aren't the ones that quickly kill human hosts. Instead, the best hosts or reservoirs for vector-borne diseases would be chronically ill over the long term, increasing the likelihood that the pathogen could be transmitted to others.

Read up on the basics of vaccinations with this 101 from the World Health Organization, the public health arm of the United Nations.

There are several different types of vaccines. Each type is designed to teach your immune system how to fight off germs—and the serious diseases they cause. This short overview breaks down the differences between the types.

Watch this under-3-minute video explaining how vaccines use the body's immune system to prepare our bodies against viral and bacterial threats to our health.

Since a landmark study of the common cold in 1925, researchers have learned much more about the seasonal ailment. Despite continuous study, people still contract colds for similar periods of time at the same rate with the same symptoms, and there's no vaccine. This article explains how the many viral strains make it hard to control.

In 2025, certain eastern states including Virginia and West Virginia reported a sizable uptick in their HFMD cases, particularly in their schools and daycare centers, though the reason for the larger outbreaks remains unknown. When adults or teens get the disease, their symptoms are usually mild or even asymptomatic. Still, they may pass the disease between people, too.

The live virus vaccine, delivered via oral drops, induces the best protection in the body. Because it includes a live, weakened form of the virus there's an extremely small risk it can sicken recipients and potentially cause paralysis. Recipients also shed the virus in their stool, which can rarely lead to vaccine-derived cases of the virus if the virus mutates.

In 18th- and 19th-century outbreaks, some New England families lost as many as eight children to diphtheria, and fatality rates in hospitals often topped 60%. The invention of antitoxins, penicillin, and vaccines dramatically reduced deaths.

The animals can contract the virus via infected mosquito bites, but for horses there are four vaccines available. Though West Nile's often asymptomatic or mild in horses, the disease can sometimes be deadly, killing via severe neurological inflammation. There are no reported instances of people becoming sickened by infected horses or vice-versa.

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.

The surge in HFMD in the summer and early fall of 2025 in the mid-Atlantic could be the result of a confluence of factors, including a more virulent strain of disease-causing virus, good environmental conditions for the virus to spread—hot, humd—and a potential immunity gap among kids due to milder HFMD outbreaks the previous year.

Germ theory of disease is based on the concept that many diseases are caused by infections with microorganisms, typically only visualized under high magnification. Read this general overview to understand the basic framework which allowed for development of vaccines.

Zoonotic diseases encompass more than 200 diseases transmitted from animals to humans, wherein humans are usually accidental hosts. Ranging from rabies to HIV to monkeypox, this overview lists some of the most common viral threats.

A major hurdle for West Nile vaccine development is the lack of a market for it since so many people are symptomatic for the infections. Still, a growing number of infections, including a 40% spike in 2025, may change that calculation.

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.

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.

Collaborations between the military, biotech, and academic researchers led to an experimental antibody treatment for the Andes hantavirus strain—the one that more readily passes between humans. As of spring 2026, it had only been tested in animals. The US Army has also tested a potential hantavirus vaccine in people. It published early findings in 2023 that seemed promising, eliciting some immune system response.

Zoonotic diseases, which spread from animals to humans, present one of the most significant emerging threats worldwide. Scientists have identified around 200 viruses capable of infecting humans, and unfortunately, the number of such diseases are on the rise—at least 60% of all emerging infectious diseases are zoonotic in origin.

Such diseases may be caused by a variety of pathogens—viruses, bacteria, fungi, or parasites—and can originate in creatures ranging from ticks and mosquitoes to pigs and bats. Because they develop in animals and are often new to human immune systems, zoonotic diseases have severe consequences for public health.

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.

The measles virus attacks the body's immune system, including preying on T and B cells in the lymphatic system. These cells hold memories of past infections, which can be degraded as the virus destroys them. This leaves patients susceptible to other infections for a year or longer.

Can the right thoughts lead to a stronger immune system? Neuroimmunologists like Hai Qi study how drugs targeting the brain, as well as thought exercises, can prompt stronger immune responses to infection. This six-minute video investigates the radical claims.

A measles outbreak in the summer of 2025 ultimately led to more than 900 cases and the deaths of two young children. The highly contagious virus requires vaccination rates near 95% to achieve herd immunity—the Texas outbreak spread through areas of significant mistrust of vaccines, including the area’s Mennonite community.

The American Animal Hospital Association provides guidelines, while many states legally require certain vaccines, such as rabies. Vaccinating healthy pets—even indoor-only pets—prevents outbreaks and protects other vulnerable animals, and adverse effects, such as allergic reactions and injection-site irritation, are rare.

One reason antibiotic resistance spreads so quickly is that bacteria have multiple mechanisms allowing them to share genetic material. This short video offers a concise explanation for each of these mechanisms, and animation that helps viewers understand how antimicrobial resistance can quickly spread from one organism to another.

Rabies, a viral disease attacking the central nervous system, is nearly always fatal once symptoms appear. It spreads through the infected animal saliva (typically through bites) and causes encephalitis, leading to severe neurological symptoms and death. See how the lyssavirus—the bugs behind rabies—work once inside the body.