List of Most Contagious Diseases

Infectious diseases have played a powerful role in shaping human history and still affect lives today—from sweeping pandemics that span continents to everyday outbreaks in classrooms and communities. What makes some illnesses spread like wildfire while others fizzle out? A key factor is the basic reproduction number, or R₀ (pronounced “R naught”)—a scientific measure of how many individuals, on average, one infected person will pass a disease to in a fully susceptible population. The higher the R₀, the more contagious the disease. This list explores the world’s most contagious diseases—those with high R₀ values and rapid transmission rates—that continue to challenge public health systems and underscore the importance of prevention, vaccination, and global vigilance.

Measles

Measles is one of the most contagious diseases in the world, with a basic R₀ value ranging from 12 to 18, meaning that a single infected person can spread the measles virus to as many as 18 others in a susceptible population. The disease is caused by the measles virus, which spreads through droplets that are released into the air by coughing and sneezing. The virus can linger in the air for as long as two hours, increasing its chances of transmission to a new host. Despite its severity, measles is preventable through the MMR vaccine, which protects against measles, mumps, and rubella. High vaccination coverage—typically 95 percent or more—is essential to achieve herd immunity and prevent outbreaks.

Pertussis (whooping cough)

Pertussis is a respiratory disease caused by Bordetella pertussis, a highly infectious bacterium that is transmitted in respiratory droplets when an infected person coughs or sneezes. Infection is particularly dangerous for infants and young children. With a basic R₀ ranging from 12 to 17, pertussis ranks among the most infectious human diseases. Early symptoms resemble a common cold but can progress to severe coughing fits followed by a characteristic “whoop” sound, giving the disease its other name, whooping cough. Vaccination is the primary prevention strategy, via the DTaP vaccine for children and Tdap booster for adolescents and adults.

Chickenpox

Chickenpox, caused by the varicella-zoster virus (VZV), is a contagious disease that produces a characteristic rash, marked by blisters that itch and scab over and flulike symptoms. The virus is spread through droplets in the air and through direct contact with the rash. The basic R₀ for chickenpox is about 10 to 12. The most effective strategy to prevent chickenpox is the varicella vaccine, which provides strong protection and has significantly reduced cases and hospitalizations in countries with widespread immunization. Handwashing and other hygiene measures, as well as avoiding contact with infected individuals, can also help limit the spread of VZV.

COVID-19

COVID-19 is a highly contagious respiratory disease caused by the virus known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), first identified in late 2019. It spreads primarily through respiratory droplets and aerosols, as well as contact with contaminated surfaces. The basic R₀ has differed by SARS-CoV-2 variant—ranging from 2–4 for the original strain to 8–10 or higher for the Omicron variant, making it significantly more transmissible than many other viruses. Prevention strategies include protection with COVID-19 vaccines and booster doses, wearing masks, handwashing, ventilation, and social distancing, especially during outbreaks or in high-risk settings.

Mumps

Mumps is a viral infection that is primarily known for producing painful swelling of the salivary glands, particularly the parotid glands near the jaw. The causative virus is spread through respiratory droplets, saliva, and close contact with an infected person. Mumps has a basic R₀ of 4 to 7, indicating moderate to high transmissibility. While often mild, mumps can lead to complications such as meningitis, encephalitis, hearing loss, or inflammation of the testes (orchitis) or ovaries (oophoritis). Mumps can be prevented with the MMR vaccine, which is highly effective when the two recommended doses are received.

Norovirus

Norovirus is a highly contagious virus and one of the leading causes of acute gastroenteritis. Outbreaks often occur in schools and healthcare settings and on cruise ships. It spreads rapidly through the fecal-oral route, often via contaminated food or water, contact with contaminated surfaces, or close person-to-person contact. With R₀ values estimated between 1.6 and 3.7, even tiny amounts of norovirus can quickly infect large numbers of people. There is no vaccine against norovirus, and because the virus is resistant to many common disinfectants as well as heat, meticulous hygiene—for example, thorough handwashing, disinfection of contaminated surfaces, and avoiding food preparation while symptomatic—is key to controlling its spread.

Are you a student?

Get a special academic rate on Britannica Premium.

Subscribe

Tuberculosis

Tuberculosis is an infectious disease caused by the bacterium Mycobacterium tuberculosis, which most commonly affects the lungs. It spreads through airborne droplets when an infected person coughs, sneezes, or speaks and can spread widely in crowded or poorly ventilated environments. The basic R₀ for tuberculosis typically ranges from 1 to 4, depending on such factors as population density and access to healthcare. Prevention strategies include early detection and treatment, vaccination with the BCG vaccine (mainly effective in children), and infection control practices in healthcare and community settings. Prompt treatment with a full course of antibiotics is essential to cure the disease and prevent the development of drug-resistant tuberculosis.

Ebola virus disease

Ebola virus disease is a severe and often fatal illness caused by any of several different strains of ebolavirus, which spread through direct contact with the blood, secretions, or other bodily fluids of infected individuals or through contact with contaminated objects or surfaces. Ebola’s transmission through close contact makes it highly dangerous in healthcare settings. The basic R₀ for Ebola is 1.5 to 2.5, though it can be significantly higher, depending on the outbreak and local conditions. Prevention of Ebola centers on strict infection control measures, including the isolation of infected individuals, the use of protective equipment by healthcare workers, and safe burial practices. For people at risk of exposure during Ebola virus disease outbreaks, vaccination with rVSV-ZEBOV can help prevent transmission and facilitate outbreak control.

Hepatitis A

Hepatitis A is a liver infection caused by the hepatitis A virus (HAV), which is typically transmitted through the fecal-oral route, most often via contact with contaminated food or water. Hepatitis A has a basic R₀ that ranges from 1.4 to 2.7. Although moderately contagious, it can spread quickly in areas with poor sanitation. Vaccination against HAV can prevent the disease; the vaccine is highly effective and is recommended for children, travelers, and at-risk individuals, such as those who lack access to clean water or who share a household with an infected individual. Additional preventive measures involve handwashing, safe food and water practices, and safer-sex practices.

Influenza

Influenza, commonly known as the flu, is a contagious respiratory illness caused by influenza viruses that infect the nose and throat and, in some cases, the lungs. The viruses are spread through airborne droplets that are produced when infected individuals cough, sneeze, or talk, as well as through contact with contaminated surfaces. The basic R₀ for seasonal influenza typically ranges from 1.3 to 1.8; thus, each infected person can pass the virus to one or two others, on average. Annual vaccination is the most effective prevention strategy, as strains of influenza viruses evolve frequently. Additional measures, including hand hygiene, wearing a mask, and staying home when sick, help reduce transmission, especially during peak flu season.