Contagious diseases are illnesses that can spread from person to person, often quickly and easily. Some are caused by viruses, others by bacteria, and a few by fungi or parasites. In the United States, many contagious diseases are now rare thanks to vaccines, improved hygiene, and public health efforts. But others still pose a serious risk—especially to children, older adults, and people with weakened immune systems.
Understanding how these diseases spread, what symptoms to watch for, and how they can be prevented or treated can help protect you and those around you. In this article, we’ll take a closer look at some of the most contagious diseases still affecting people in the U.S., how they spread, and what you can do to lower your risk.
What Makes a Disease Contagious?
A contagious disease is one that spreads from person to person through direct or indirect contact. That might mean touching an infected person, breathing in germs from their cough, or touching a surface that has the virus or bacteria on it.
Some diseases are more contagious than others. For example, measles spreads more easily than the flu. How contagious a disease is depends on the type of germ that causes it, how it’s transmitted, and how long someone is contagious. In some cases, a person can spread illness before they even feel sick themselves.
Public health experts measure how contagious a disease is by something called the reproduction number, or R0 (pronounced “R-naught”). The higher the number, the more easily it spreads. But even less contagious diseases can become serious if they aren’t treated or prevented.
Common Highly Contagious Diseases in the U.S.
Many diseases that spread easily are viral, though some are caused by bacteria. Some cause mild symptoms, while others can lead to serious complications. Let’s look at a few of the most common ones.
1. Influenza (Flu)
The flu is a viral infection that spreads mainly in the fall and winter. It travels through the air when someone coughs or sneezes and can also live on surfaces. The flu causes fever, chills, cough, sore throat, and fatigue. Most people recover in a week or two, but for older adults and people with health conditions, it can be dangerous.
2. Measles
Measles is a highly contagious viral disease that spreads through the air. Just being in the same room as someone with measles can lead to infection. Symptoms include fever, cough, runny nose, and a red rash that starts on the face. Measles can cause serious problems like pneumonia and brain swelling, especially in children who haven’t been vaccinated.
3. Norovirus
This virus causes vomiting and diarrhea and is often called the “stomach flu,” even though it’s not related to influenza. It spreads easily through contaminated food, water, and surfaces, and outbreaks are common in places like cruise ships, schools, and nursing homes. It’s very contagious—even tiny amounts of the virus can make you sick.
4. Chickenpox (Varicella)
Chickenpox spreads through the air and by touching the fluid from blisters. It causes an itchy rash, fever, and tiredness. It’s most common in children, but adults who haven’t had it before can get very sick. Since the chickenpox vaccine became widely used, the number of cases in the U.S. has dropped a lot.
5. Whooping Cough (Pertussis)
This bacterial infection causes severe coughing fits and is especially dangerous for babies. It spreads through coughing and sneezing. Early symptoms look like a cold, but after a week or two, the cough gets worse and can last for weeks. Vaccination is the best way to prevent it.
6. Common Cold
The cold is caused by many different viruses and is very easy to catch. It spreads through the air and by touching surfaces. Most people get better on their own within a few days, but it can still be miserable—causing a runny nose, sore throat, sneezing, and coughing. Because there are so many viruses that cause it, there’s no vaccine.
How These Diseases Spread
Contagious diseases spread in several ways. Some move through droplets when a sick person coughs or sneezes. Others are airborne and can stay in the air longer or travel farther. Some spread by touching contaminated surfaces or by sharing food, drinks, or personal items. Infections like norovirus can also be spread by contact with stool or vomit, especially in group settings.
Washing your hands regularly, avoiding close contact with sick people, covering coughs and sneezes, and staying home when you’re sick can all help reduce the spread of germs. Vaccines are also one of the most powerful tools for preventing many contagious diseases.
Symptoms to Watch For
While each illness has its own symptoms, there are some general signs that could mean you’re contagious or getting sick:
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Fever or chills
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Cough or sore throat
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Runny or stuffy nose
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Headache or body aches
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Nausea, vomiting, or diarrhea
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Rash or skin changes
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Feeling unusually tired or weak
If you notice these symptoms—especially if they come on suddenly—it’s a good idea to stay home, rest, and call a doctor if needed.
Treatment and Prevention
Most viral infections go away on their own with rest, fluids, and over-the-counter medications for symptoms. Some, like the flu and COVID-19, have antiviral treatments that can help if started early. Bacterial infections like strep throat or whooping cough may need antibiotics. But not all infections need medicine—and in some cases, antibiotics won’t help and could even cause harm.
The best way to protect yourself and others is through prevention. This includes keeping your hands clean, disinfecting surfaces, avoiding close contact with sick people, and staying up to date on recommended vaccines.
