What Are Pathogens?

Pathogens are what most people call germs—tiny organisms that cause diseases. There are different types of pathogens found all over the world. Depending on the type of germ and your body, you can get a minor illness or a deadly disease when a pathogen enters.

This article explains four of the most common types of pathogens. It also offers suggestions on how you can protect yourself from diseases they cause.

A doctor looking at a dish full of pathogens
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How Pathogens Infect the Body

Germs are divided into categories. The most common types are:

  • Bacteria
  • Viruses
  • Fungi
  • Parasites

These pathogens are different in many ways, but they have one main similarity: Once they’re inside you, they can damage cells or interfere with the body’s normal activities.

Pathogens can enter your body (which scientists call “the host”) in several ways:

  • Breathing them in: If a sick person sneezes or coughs, they send pathogens into the air. You could then inhale those germs.
  • Contact with blood: You may be exposed to infected blood during sexual contact, touching a person’s wound, or even from blood-sucking insects.
  • Food and water: Eating or drinking something infected with pathogens can make you sick.
  • Physical contact: If you touch someone or something that has germs and then touch your mouth, nose, eyes, ears, or a cut, that pathogen enters your body.

The fecal-oral route refers to germs being passed on from one person’s stool to another person’s mouth. For instance, if you change a sick baby’s diaper and touch your hands to your face before washing, you may catch whatever infection the baby has. Something similar happens when there are unsanitary bathroom conditions.

Not all viruses, bacteria, fungi, and parasites are pathogens. Instead of causing disease, some help the body. For instance, “good bacteria” in yogurt may lower your risk of type 2 diabetes and heart disease.


Most bacteria are actually harmless, but can cause bacterial infections. Often, these germs make toxic chemicals. As the bacteria quickly multiply, their toxins kill or damage healthy tissue.

Examples of bacterial infections include:

If a certain infection is confirmed and likely to progress and/or cause harm quickly, your healthcare provider will prescribe antibiotics right away. In other cases, they may take a “wait and see” approach, giving the infection a chance to clear up on its own without medication in an effort to avoid antibiotic resistance.

Antibiotic resistance happens when someone uses antibiotics too often. The medications can no longer effectively fight the bacteria because the pathogens have essentially outsmarted them.

One example of this is methicillin-resistant Staphylococcus aureus (MRSA), which is a staph infection that has become immune to antibiotics normally used to treat other infections in the staph family.


Viruses need to live on and feed on something. Unfortunately, what they live and feed on is you. Smaller than bacteria, these germs spread easily from person to person.

Examples of viral infections range from mild to severe and include:

Treatments for viruses depend on the infection type. In general, treatments won’t kill the virus in your body. Medication, rest, and building your immunity, though, can help you deal with symptoms while you wait for the virus to run its course or for a flare-up to end.

There are three main goals of virus treatment:

  1. Manage symptoms: Option may include pain relievers to soothe aches, gargling with salt water to help with a sore throat, decongestants to improve a stuffy nose, and using a chest rub to ease a cough.
  2. Reduce flare ups: Some viruses like herpes live in your body forever, but they only cause problems at certain times. There are medications called antivirals that can lower the risk for an outbreak, which would bring on symptoms again. Other outbreak risks include stress, illness, and sunburn.
  3. Lower how much of the virus is in your system: This is called reducing your viral load. Taking medications or following practices that boost your immune system can make you better able to fight viruses. You may not kill the virus, but if you’re healthy and well-defended, it may limit how much the virus is able to spread in your body.

Some viral infections, like influenza or HIV, can increase your risk of developing a bacterial infection.


Fungi live in the air and soil, on plants, and in water. There are different types of fungi in the world are harmful to people. When fungi in nature reproduce, they send spores into the air. If you inhale the spores or get them on you, they can get into your lungs or on your skin where they can grow and cause problems like fungal pneumonia or a fungal rash.

There are three types of fungal infections:

  • Superficial: These usually affect the groin, hands. Jock itch, athlete’s foot, and ringworm fall into this group.
  • Subcutaneous: This affects the tissue in and beneath the skin. Sporotrichosis is one common type that affects people who garden. It causes bumps on the skin that can develop into open sores.
  • Systemic: This type of infection can affect the blood, eyes, and heart. For instance, a common fungus can cause cryptococcosis. For people with compromised immune systems, cryptococcosis can lead to a brain infection.

Having an organ transplant or HIV weakens your immune system and makes you more likely to get any type of fungal infection. Taking antibiotics also puts you at risk.

If you have a life-threatening fungal infection, your doctor may prescribe a very strong antifungal medication (fungicide). These drugs, such as amphotericin, have serious side effects but may be necessary for major infections.


Parasites may be simple, one-celled organisms. In all cases, they cause illness while feeding on your cells. Some parasites that cause illness include:

  • Protozoa that causes malaria through a mosquito or sand fly
  • Helminths that cause intestinal worms
  • Ectoparasites that cause infections through ticks, fleas, lice, and mites

Doctors treat parasites with anti-parasitic drugs. The type of parasite determines which types of medications can be used. Your doctor also needs to consider your overall health, age, weight, and other factors.

In some areas of the world, different parasites are resistant to one type of treatment. This is an issue for malaria treatment in several places, for instance. Alternative medications then need to be considered.  

Defending Against Pathogens

Your body is equipped with amazing ways to defend itself against pathogens. Nose hair, for instance, acts as a filter that stops some germs from getting into the body.

A healthy immune system, though, is the best defense you have against germs. Focus on everyday healthy habits—eat well, exercise, and get enough sleep—to support its efforts.

Also commit to these effective strategies for staying well:

  • Wash your hands with soap and water or an alcohol hand rub.
  • Avoid close contact with people in public. This is especially important if you already have a health problem or illness.
  • Wear a mask in crowded spaces and when around ill individuals.
  • Don’t touch your face after touching a public surface like a doorknob or bathroom faucet.
  • Stay up-to-date on all of your vaccinations.

And remember that protection is a two-way street: If you are sick, do what you can to keep others safe by covering a cough and staying home from work when necessary.


Bacteria, viruses, and fungi are all types of parasites that can cause minor or severe health issues when they enter your body. Each of these pathogens includes a variety of subtypes that are responsible for a wide range of infections or diseases.

To avoid complications related to these illnesses, do your best to prevent infections in the first place. Building a healthy immune system and knowing how to avoid situations that put you at risk for pathogen exposure are the most effective ways to stay healthy.

Frequently Asked Questions

  • How do viruses differ from other pathogens?

    Viruses differ from other pathogens in that they have only one purpose—to get inside a host and replicate itself. That host can be a human or any living thing, including bacteria and fungi. Viruses are extremely simple pathogens that are even smaller than bacteria.

  • Are all bacteria pathogenic?

    No, not all bacteria are pathogenic. In fact, most are considered helpful and essential to human life. For example, the bacteria that reside in our intestines, known as gut flora, help us digest food. Other types of bacteria help us fight disease, such as pathogenic bacteria.

  • How many pathogens are there?

    There are about 1400 human pathogens that have been identified, but there are believed to be many more which have not yet been discovered. This total amount is made up of the four main categories of pathogen (bacteria, viruses, fungi, and parasites).

  • Can pathogens cause cancer?

    Yes, certain pathogens can cause cancer. A few examples of these include Epstein-Barr Virus, hepatitis B, hepatitis C, and human papillomaviruses. Additionally, they can weaken the immune system or disrupt normal bodily functions leading to an increased risk of cancer. A few ways to prevent infection from many pathogens is by using protection during sex, receiving vaccinations, and avoiding sharing needles.

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By Ingrid Koo, PhD
 Ingrid Koo, PhD, is a medical and science writer who specializes in clinical trial reporting