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What Are Virus Variants?

Learn how viruses mutate to create variants, such as Delta and Omicron, and how vaccines work against variants.


What is a Virus?

Viruses occupy a gray area between the living and nonliving realms. Scientists have identified more than 200 virus species with the potential to infect humans.


How Do Viruses Infect Our Cells?

New viruses are discovered every year, and some, such as SARS-CoV-2, cause disease. Explore the mechanics behind infection.


How Do Viruses Spread?

Learn about the multiple pathways through which virus transmission can occur and how SARS-CoV-2, the virus that causes COVID-19, is spread between people.


How Do Masks Work?

While face coverings primarily prevent the wearer from spreading the disease, they also can protect the wearer from others. Two lines of evidence support the effectiveness of face coverings in reducing the spread of COVID-19.


How Do Vaccines Work?

Vaccines are powerful and effective tools for preventing and slowing the spread and reducing the severity of disease.


How Are Vaccines Developed?

Careful testing is necessary to ensure new vaccines are safe and effective. Learn how scientists create vaccines, and why the process takes as long as it does.


What Is Passive Immunization?

Find out how antibodies from other sources can be used to provide short-term protection against infection.


How Does COVID-19 Testing Work?

Accurate testing is necessary to better control the spread of diseases like COVID-19. Learn the difference between PCR, antigen, and antibody tests.

Fighting Viruses

How Do Antivirals Work?

Antivirals are medications used specifically to treat viral infections. Learn how scientists develop treatments for diseases such as COVID-19.


How Does Disease Modeling Work?

Models help policymakers forecast the spread and severity of disease. How do researchers create them?


Ask a Caltech Expert

Experts in immunology, aerosols, materials science, and more answer the public's questions about science-based solutions to the coronavirus pandemic. Learn about booster shots, antibody treatments, and masks.


Terms to Know

Also known as immunoglobulin (Ig), a protein in the blood and component of the immune system used to fight pathogens (microbes that cause illness).

A test that looks for an antibody response to a specific pathogen, indicating that an individual has previously been infected. Learn more about antibody testing.

A foreign substance that triggers an immune response in the body.

A type of therapeutic used to treat a viral infection. Antivirals do not work against bacteria just as antibiotics do not work against viruses.

Bivalent vaccines work by triggering an immune response to two antigens. The bivalent COVID-19 booster contains two messenger RNA (mRNA) components of SARS-CoV-2: one of the original strains of SARS-CoV-2 and one from the BA.4 and BA.5 lineages of the Omicron variant of the virus. This updated booster provides protection that is better matched to the currently circulating virus variants. The original COVID-19 vaccines administered in 2020, are "monovalent."

A dose of a vaccine given after initial immunization, which is called a prime. The additional dose is called a booster because it is meant to boost the recipient's immune system response to a specific pathogen.

A detectable viral or bacterial infection after full vaccination to prevent disease caused by that infection.

A process used to identify people who may have come into contact with a person infected with a disease and are thus at a higher risk of becoming infected themselves.

A family of viruses that have a crown-like appearance and cause illnesses ranging from the common cold to severe diseases such as Middle East Respiratory Syndrome (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS-CoV).

The disease caused by the novel coronavirus, SARS-CoV-2. COVID-19 is short for "Coronavirus Disease 2019."

Resistance to the spread of a contagious disease within a population as a result of a high proportion of individuals developing immunity.

Having an immune system less able to recognize and defend against pathogens and to gain the full protective effect of vaccines because of medical conditions, treatments, or genetic factors.

The time between exposure to a virus and the emergence of symptoms or signs of illness.

In the context of disease, it is the act of isolating oneself from others due to the presence of symptoms. It is a means of controlling the spread of disease.

Also known as messenger RNA, a type of genetic material that instructs the cellular machinery that makes proteins. mRNA serves as the "messenger" between DNA and the protein-making machinery.

One of a group of "nucleic acid amplification tests" that looks for the presence of viral genetic material in the body. They are used to diagnose active infections. Learn more about PCR testing.

To keep someone or something apart from others after exposure to a contagious disease, such as COVID-19, to prevent the spread of infection. The length of and instructions for quarantining differ between diseases and depend on how each disease is spread. Quarantine differs from isolation, in which people who are sick and/or contagious are separated from those who are not sick.

The practice of maintaining distance from or avoiding physical contact with others in an attempt to prevent the spread of disease.

A protein that projects out from the surface of some viruses and is necessary for viral entry into a host cell.

Viruses reproduce by taking over the replication machinery of host cells to make copies of their own genetic material. Often, this process results in errors, known as mutations, which make the new virus different from the original. The mutated version is called a variant.

Dive Deeper

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