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12.3. Infectious diseases

Infectious diseases

The infectious diseases are caused by pathogenic microorganisms or substances produced by them. These microscopic beings can be made up of one or more cells or even be acellular.

  • Bacteriaprokaryotic unicellular beings, without a nuclear membrane, causing diseases such as diphtheria, salmonellosis, pneumonia, tuberculosis, cholera and tetanus, among others.

Activity: Classification of bacteria by their shape.

Song: Yes, I know you are remembering the song "I am a bacterium".

  • Fungi: Eukaryotic and heterotrophic unicellular or multicellular organisms. They live in different environments, such as:
    • Saprophytic fungi: They live in humid places on dead organic matter.
    • Parasitic fungi: They live inside or outside of other living beings, which they harm. They are disease-causing fungi.
    • Symbiotic fungi: They live associated with algae, forming lichens.

Micosis. Enfermedades producidas por hongos

By Bobjgalindo (Own work) [GFDL or CC BY-SA 4.0-3.0-2.5-2.0-1.0], via Wikimedia Commons

Examples of diseases (mycoses) are fungal ringworm and candidiasis.

  • Protozoa: They are eukaryotic unicellular organisms that live in water and some, in liquids such as blood. They are the cause of diseases such as malaria and sleeping sickness.
  • Viruses: They are not true living beings, as they are acellular. They are obligate parasites and always need to take advantage of a cell's structure and metabolism in order to reproduce. Some examples of diseases caused by viruses are rubella, chickenpox, measlesflu, and AIDS.

Activity: Diseases according to the type of microorganism that causes them.

Answer in your notebook

12.4.- Are all microorganisms pathogens?. Give reasons for the answer.

Infectious disease transmission

Infectious diseases are transmitted by an agent (the pathogen) carried by a vector (animals that carry the pathogen and do not have the disease, such as a mosquito) to the host (the person who contracts the disease) .

 The transmission of infectious diseases can be done by:

  • Direct contact with other people. Microorganisms are transmitted from a sick person to a healthy person by direct physical contact, such as touching, kissing, having sex, sneezing, etc. For example, syphilisgonorrhealeprosysmallpoxchickenpoxmeasles, etc .


By Thomas Netsch [Public domain], via Wikimedia Commons

  • Indirect contact. The disease is transmitted from the sick person to a healthy person without direct contact between the two, occurring through contaminated elements, such as soil, air, water, food, contaminated objects such as syringes or scalpels.

The pathogenic microorganisms can enter the host (the healthy person) through several routes:

  • Cutaneous route. The microorganism can enter through wounds on the skin or through insect bites or animal bites. For example, the tetanus, the rage, the malaria, etc .
  • Airway. Through the air we breathe, for example, the germs in the small drops of saliva from people who cough or sneeze that can transmit fludiphtheriatuberculosis, ...
  • Digestive tract. They enter by ingesting contaminated food or water. They produce diseases such as salmonellosis or cholera.
  • Genital route. Sexual contact is the transmission mechanism for sexually transmitted diseases such as gonorrhea  or AIDS.


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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.