First, we must have a clear definition of pathogens: A pathogen is a living organism that causes diseases.

Pathogens are not just viruses ,bacteria and fungi – other types of pathogens can infect many living organisms. Sometimes bacteria themselves are infected with a pathogen.

There are many pathogenic agents in the world. Experts say that the number of viruses in our planet is even more than the stars in the whole universe. You don't need to worry too much about micro-organisms, since only a small portion of them are harmful to health. Some of them are even helpful!

Types of pathogens

As we mentioned in the previous section, only bacteria and viruses cannot cause diseases. There are other similar creatures, including:

Fungi: Fungi cause a type of infection in athletes' feet (fungal infections)

Protozoa: These large single-celled organisms cause malaria.

Parasites, lice and tapeworms are examples of common parasitic infections.

How do pathogens cause diseases?

1-     Optional pathogens: these types of pathogens can multiply in the host or on a surface at home

2-     Obligate pathogens: these types of microorganisms reproduce only in one host (host cells). For this reason, all viruses are included in this category because they cannot reproduce alone like bacteria or fungi.

3-     Some pathogens need multiple hosts to reproduce.

4-     Some pathogens live only in one type of host

5-     Other pathogens can make a wide range of organisms sick. During this time of corona epidemic, tigers have been infected with corona virus after contact with their human caretakers (viral infection)

6-     Two other viruses, including bird flu and swine flu, cause disease in both humans and animals.

7-     Pathogens that cause disease in humans and animals are called common diseases between humans and animals.

How do pathogens cause diseases?

Pathogenic agents cause disease in different ways. Sometimes they damage tissues by producing toxins.

In some cases, the pathogen mounts a strong response that damages healthy tissue along with infected tissue. (infectious diseases)

Pathogens use this response to spread. Sneezing, coughing and diarrhea cause pathogens to find a new host for infection.

How do pathogenic agents multiply?

These pathogens spread through humans. For example, it is possible to cough or sneeze into your hands, then touch surfaces and other people. You can also transfer these pathogens to food. Also, pathogenic agents present in raw materials can spread to other foods in the kitchen.

Prevention of pathogenic infections

Definitely, the most effective way is to wash your hands with soap and water or use disinfectants.

Alcohol-based disinfectants can cause skin damage because they kill the good bacteria on the skin.

These bacteria protect against harmful agents, and if they are destroyed, it creates an environment where harmful agents can flourish. It also causes skin damage such as eczema, itching, etc.

• We suggest you read the article Effects of using high percentage alcohol disinfectants

Beneficial microorganisms

As we said, not all microorganisms are harmful, but some of them are very useful. In fact, a microorganism is considered a pathogen when it causes disease.


Fungi are very important in the nature. They are one of the biggest decomposers of dead organisms.

One of the most useful fungi in nature is Penicillium notatum, which is used to make the antibiotic penicillin.


Some protozoa are useful for purifying water or keeping soil healthy.


You might be surprised to know that your gut contains bacteria that keep the body healthy and digest food.


Yes that's right! Viruses can also be useful. Today, scientists use viruses to treat some diseases through gene therapy.

They change the genetics of a virus so that it is no longer harmful. Then they use the ability of this virus to infect cells to carry useful DNA to the body. In fact, this new DNA makes cells make new proteins and helps with genetic disease.