May 2003

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How contagious are they?

back to SARS article

Daniel Musher, MD, an infectious disease researcher at the Houston VA Medical Center and a professor of medicine and molecular virology and microbiology at Baylor College of Medicine, wrote “How Contagious are Common Respiratory Tract Infections,” in the March 27 issue of The New England Journal of Medicine. Following is a ranking of the contagiousness of some common illnesses.

Neisseria meingtitidis - causes meningitis
Moderately contagious - Despite public fear, often bordering on hysteria that may follow a case of meningitis, more than 95 percent of cases of meningitis in the United States and other developed countries are sporadic. When a single case occurs in a schoolchild, the Centers for Disease Control and Prevention recommends treatment for immediate family, but not schoolmates. The infection is spread person to person by direct physical contact or the inhalation of droplets that carry the microorganisms.

Streptococcus pyogenes - causes conditions including strep throat, scarlet fever, impetigo and cellulitis
Moderately contagious in families or close populations - School-age children are most likely to bring the contagion into the family. The infection is spread by intimate contact through the inhalation of or contact with large-droplet secretions. In a family, the risk of contagion of streptococcal pharyngitis is about 10 percent.

Streptococcus pneumonie - causes pneumonia
Generally, not very contagious - The disease occurs when organisms are not cleared from the lungs, sinuses or middle ear through normal means. Cigarette smoking, pollution, allergies and viral infection may interfere with the body clearing itself of the organisms. The organisms then can populate the body’s air spaces before it has a chance to produce an antibody. Contagion is also more common in populations with a weakened immune system, or among people who are unvaccinated and lack an antibody to the organism.

Mycobacterium tuberculosis - causes tuberculosis
Very contagious - Tuberculosis infects nearly one-third of the population of the world, which amounts to about 2 billion people. TB kills almost 3 million people annually. The illness is highly contagious in families, schoolrooms, dormitories, nursing homes, assisted-living facilities, shelters and bars. Most persons who are infected with M. tuberculosis harbor the bacterium without symptoms but many develop active TB disease. The disease is spread primarily through inhalation of the bacteria, which is often expelled when a TB patient coughs.

Influenza viruses - causes the “flu”
Very contagious - Influenza is a viral respiratory tract infection and is more contagious than bacterial infections. Transmission occurs through aerosol or direct contact. Inhalation of as few as three infective particles can transmit infection. The majority of infected persons also have symptoms of the disease, which can enhance the likelihood of contagion.

Rhinoviruses - causes the “common cold”
Very contagious - The introduction of rhinovirus into a household by one family member causes the disease in about two-thirds or of other family members. The illness is spread both by direct contact and by aerosol.

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