Many infectious diseases are caused by viruses and bacteria.

Bacteria are microorganisms that are made up of a single cell. They can be found in a wide variety of environments. Most bacteria are harmless and don’t cause disease in people. In fact, you have large numbers of beneficial bacteria living in your digestive tract that help you digest your food.

There are some instances when bacteria can cause disease in people. These bacteria are referred to as pathogenic bacteria. Bacterial diseases that you may recognize include:

Pathogenic bacteria are infectious, meaning that they can enter your body and begin to cause disease. However, not all bacterial pathogens are contagious. Contagious means that a disease can spread from person to person.

Read on to learn more about bacterial infections, which types are contagious, and how they spread.

The amount of time that a bacterial infection is contagious can vary depending on what type of bacteria is causing your illness.

When do you begin to be contagious?

For some infections, such as strep throat and whooping cough, you’re considered to be contagious when you start to experience symptoms.

Other infections, such as chlamydia, can be asymptomatic, meaning they don’t present symptoms. For this reason, you could transmit these infections to other people without knowing it.

When are you no longer contagious?

Antibiotics are often used to treat bacterial infections. These medications specifically target bacterial functions and can either kill bacteria or prevent them from thriving.

You’re typically considered no longer contagious after you’ve been on a regimen of antibiotics for a period of time, which depends on your type of infection.

For example, you’re no longer contagious with strep throat after you’ve been on antibiotics for 24 hours and no longer have a fever.

Additionally, you’re no longer contagious with whooping cough after five full days on antibiotics. People with chlamydia should abstain from sexual activity until they’ve completed seven days of antibiotic treatment.

It’s very important to speak with your doctor about your infection and how long you should expect to be contagious. Knowing this information can help prevent you from infecting others while you recover.

Bacterial infections can be acquired in several different ways, depending on the type of infection. Let’s explore some examples of how some bacterial illnesses are spread.

Whooping cough

Whooping cough, or pertussis, is a very contagious respiratory illness. The bacteria that causes it can be expelled in respiratory droplets that are formed when an infected person coughs or sneezes.

If you inhale these droplets, you may become infected. Touching contaminated objects such as doorknobs can also spread the infection.


Impetigo is a very contagious skin infection. The infection can be acquired by direct skin-to-skin contact with an infected person. You can also get it by using an object, such as a towel, that’s been contaminated with the bacteria.


Cellulitis is a bacterial skin infection that’s infectious but not usually contagious. You can get cellulitis when bacteria that are normally present on the surface of your skin invade the deeper layers of your skin through something like a cut, scrape, or burn.


Salmonella is a type of foodborne illness. People with salmonella can be contagious, as the bacteria can spread through feces. People with the infection who don’t follow proper hygiene procedures can spread the bacteria to objects and food.

Animals such as chickens, cows, and reptiles also carry salmonella. You can become infected if you come into contact with these animals and don’t wash your hands afterward. You can also acquire the bacteria through contaminated meats, eggs, or milk.


Chlamydia is a common contagious sexually transmitted infection (STI). It can be spread through coming into sexual contact with someone who has it.

The bacteria can also be spread from mother to child during childbirth.

Lyme disease

Lyme disease is an infectious bacterial disease that’s spread to humans through the bite of an infected tick. It doesn’t spread from person to person.

It depends.

The overall contagiousness of a disease involves many factors, including:

  • how many people in the population are susceptible to the disease
  • the amount of time an infected person is contagious
  • how many people an infected person is likely to come into contact with
  • how the disease is transmitted

Viruses are very tiny microorganisms that are even smaller than bacteria. They invade the cells of your body where they then use cellular components to replicate themselves. Some viral diseases that you may be familiar with include:

Measles, an airborne viral disease, is the most contagious infectious disease. A person with measles is able to infect anywhere between 12 to 18 additional people in a susceptible population.

In contrast to this is Ebola, a viral disease that’s transmitted via contact with the bodily fluids of an infected person. Someone with Ebola may infect about two additional susceptible people.

Whooping cough is the most contagious bacterial infection. Like measles, it’s primarily spread through the air. An infected individual can potentially infect anywhere between 12 to 17 other susceptible people.

Comparatively, a person infected with diphtheria, another bacterial infection that can be spread through airborne droplets, may only infect six to seven susceptible individuals.

As you can see, the overall contagiousness of a disease varies, regardless of if it’s bacterial or viral.

Not all bacterial conditions are contagious. This means that they aren’t spread from person to person but are instead acquired in other ways.

Some bacterial infections that are acquired from animals aren’t contagious. These infections are often spread through the bite of an infected animal. Some examples include:

Other bacterial infections are acquired through the environment. You can get them through contaminated food, or the bacteria can enter an infected wound directly from the surrounding environment. Examples include:

  • tetanus, which can enter the body from the environment through wounds or injuries
  • botulism, which can be acquired through contaminated food or through a wound
  • hot tub folliculitis, which is caused by a bacteria called Pseudomonas and happens when you use a poorly maintained hot tub
  • tularemia, which can get into the body through contaminated food or water or through inhaling bacteria from the environment

Some bacterial conditions themselves aren’t contagious, but the bacteria that can potentially cause them are contagious.

For example, the Staphylococcus bacteria itself can be transmitted from person to person through direct skin-to-skin contact, including through contact with fluids or pus from an infected wound. It can also be acquired through contact with a contaminated object.

Once the bacteria have colonized, they can stay on your body for a few months up to several years. It’s possible to have Staphylococcus bacteria on your body and never become ill. However, the bacteria can sometimes take advantage of wounds or other breaks in the skin to enter the body and cause conditions such as cellulitis, abscesses, and folliculitis.

Many bacterial infections can be treated with a course of antibiotics, although some infections may be more serious.

It’s extremely important to finish the entire course of antibiotics your doctor prescribes for you. This not only increases the chance of eliminating the disease-causing bacteria from the body, but it also reduces the risk that antibiotics won’t be effective in the future.

Be sure to follow the tips below to reduce your risk of catching a contagious bacterial infection:

Practice good hand hygiene

Wash your hands frequently. Situations where you should always wash your hands include:

  • after using the bathroom
  • before eating
  • before and after cooking or preparing food
  • before touching your face, nose, or mouth

Don’t share personal items

Things such as toothbrushes, razors, and eating utensils can all spread disease.

Stay up-to-date on your vaccines

Many contagious bacterial infections, such as whooping cough, are preventable through vaccination.

Practice safe sex

Always use a condom if you have a new sexual partner or if your partner has a history of STIs.