Germs: Understand and protect against bacteria, viruses and infections

Learn how germs work and what you can do to protect yourself.

By Mayo Clinic Staff

Germs live everywhere. You can find germs (microbes) in the air; on food, plants and animals; in the soil and water — and on just about every other surface, including your body.

Most germs won't harm you. Your immune system protects you against infectious agents. However, some germs are difficult enemies because they're constantly mutating to breach your immune system's defenses. Knowing how germs work can increase your chances of avoiding infection.

Infectious agents: From bacteria to worms

Infectious agents come in many shapes and sizes. Categories include:

  • Bacteria
  • Viruses
  • Fungi
  • Protozoans
  • Helminths


Bacteria are one-celled organisms that can be seen only with a microscope. They're so small that if you lined up a thousand of them end to end, they could fit across the end of a pencil eraser.

Not all bacteria are harmful, and some bacteria that live in your body are helpful. For instance, Lactobacillus acidophilus — a harmless bacterium that resides in your intestines — helps you digest food, destroys some disease-causing organisms and provides nutrients.

Many disease-causing bacteria produce toxins — powerful chemicals that damage cells and make you ill. Other bacteria can directly invade and damage tissues. Some infections caused by bacteria include:

  • Strep throat
  • Tuberculosis
  • Urinary tract infections


Viruses are much smaller than cells. In fact, viruses are basically just capsules that contain genetic material. To reproduce, viruses invade cells in your body, hijacking the machinery that makes cells work. Host cells are often eventually destroyed during this process.

Viruses are responsible for causing many diseases, including:

  • AIDS
  • Common cold
  • Ebola
  • Genital herpes
  • Influenza
  • Measles
  • Chickenpox and shingles
  • Coronavirus disease 2019 (COVID-19)

Antibiotics designed for bacteria have no effect on viruses.


There are many varieties of fungi, and people eat several of them. Mushrooms are fungi, as are the molds that form the blue or green veins in some types of cheese. And yeast, another type of fungus, is a necessary ingredient in most types of bread.

Other fungi can cause illness. One example is candida — a yeast that can cause infection. Candida can cause thrush — an infection of the mouth and throat — in infants and in people taking antibiotics or who have an impaired immune system. Fungi are also responsible for skin conditions such as athlete's foot and ringworm.


Protozoans are single-celled organisms that behave like tiny animals — hunting and gathering other microbes for food. Many protozoans live in your intestinal tract and are harmless. Others cause diseases, such as:

  • Giardiasis
  • Malaria
  • Toxoplasmosis

Protozoans often spend part of their life cycles outside of humans or other hosts, living in food, soil, water or insects. Some protozoans invade your body through the food you eat or the water you drink. Others, such as the malaria protozoans, invade your body through mosquito bites.


Helminths are among the larger parasites. The word "helminth" comes from the Greek word for worm. If these parasites — or their eggs — enter your body, they settle in your intestinal tract, lungs, liver, skin or brain, where they live off your body's nutrients. Helminths include tapeworms and roundworms.

Understanding infection vs. disease

There's a difference between infection and disease. Infection, often the first step, occurs when bacteria, viruses or other microbes that cause disease enter your body and begin to multiply. Disease occurs when the cells in your body are damaged — as a result of the infection — and signs and symptoms of an illness appear.

In response to infection, your immune system springs into action. An army of white blood cells, antibodies and other mechanisms goes to work to rid your body of whatever is causing the infection. For instance, in fighting off the common cold, your body might react with fever, coughing and sneezing.

Warding off germs and infection

What's the best way to stay disease-free? Prevent infections. You can prevent many infections and avoid spreading infections through simple tactics such as these:

  • Avoid close contact with people who are sick.
  • Cover coughs and sneezes.
  • Avoid touching your face.
  • Stay home if you're sick.
  • Clean and disinfect surfaces that are touched often.
  • Avoid contaminated food and water.

You can also prevent infections through:

  • Hand-washing. Often overlooked, hand-washing is one of the easiest and most effective ways to protect yourself from germs and most infections. Wash your hands with soap and water for at least 20 seconds. Wash your hands before preparing or eating food, after coughing or sneezing, after changing a diaper, and after using the toilet. When soap and water aren't available, alcohol-based hand-sanitizing gels with at least 60% alcohol can offer protection.
  • Vaccines. Vaccination is your best line of defense for certain diseases. As researchers understand more about what causes disease, the number of diseases that can be prevented by vaccines continues to grow. Many vaccines are given in childhood. But adults still need vaccines to prevent some illnesses, such as tetanus, influenza and COVID-19.
  • Medicines. Some medicines offer short-term protection from certain germs. For example, taking an anti-parasitic medication might keep you from becoming infected with malaria if you travel to or live in a high-risk area.

When to seek medical care

Seek medical care if you suspect that you have an infection and you have experienced:

  • An animal or a human bite
  • Difficulty breathing
  • A cough lasting longer than a week
  • Periods of rapid heartbeat
  • A rash, especially if it's accompanied by a fever
  • Swelling
  • Blurred vision or other difficulty seeing
  • Persistent vomiting
  • An unusual or severe headache

Your health care provider can perform diagnostic tests to find out whether you're infected, how serious the infection is and how best to treat that infection.

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March 08, 2022 See more In-depth