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Diseases and Immunity Notes

Diseases and Immunity

A host is an organism, such as a person or animal, in which a pathogen lives and reproduces. Diseases caused by pathogens are typically transmissible, meaning they can be passed from one host to another.

10.1 Transmission of Pathogens

A pathogen is a microorganism (a tiny organism visible only with a microscope) that causes disease. These pathogens invade our bodies and multiply, leading to illness (Figure 10.2).

Types of Pathogens

Table 10.1 outlines four types of microorganisms that can act as pathogens, along with examples of diseases they cause:

  • Viruses: Cause diseases like influenza, common cold, poliomyelitis, measles, and AIDS.
  • Bacteria: Cause diseases like cholera, syphilis, whooping cough, tuberculosis, and tetanus.
  • Protoctists: Cause diseases like malaria and amoebic dysentery.
  • Fungi: Cause diseases like athlete's foot and ringworm.

Once inside the body, pathogens damage cells in several ways:

  • Some pathogens live inside cells and consume their resources.
  • Others produce toxins, which are waste products that harm cells and body systems. These toxins circulate in the body, causing symptoms like high temperature and rashes.

Some toxins, like the one produced by Clostridium botulinum, are extremely dangerous.

Key Terms:

  • Pathogen: A microorganism that causes disease.
  • Host: An organism in which a pathogen lives and reproduces.
  • Transmissible disease: A disease that can be passed from one host to another and is caused by pathogens.
  • Toxin: A poisonous substance or chemical that damages cells.
  • Symptoms: Features that you experience when you have a disease.

How Pathogens Enter the Body

Transmission is the movement of a pathogen from one host to another. Infection refers to the entry of the pathogen into the body of a host.

Key Terms:

  • Transmission: The movement of a pathogen from one host to another.
  • Infection: The entry of a pathogen into the body of a host.

Direct Contact

Some pathogens are transmitted through direct contact between an infected person and an uninfected person. Diseases spread in this way are known as transmissible diseases (or contagious diseases).

  • HIV: The human immunodeficiency virus, which causes AIDS, can be transmitted when an infected person's blood comes into contact with another person's blood.
  • Athlete's foot: The fungus causing this skin infection can be passed on by sharing a towel with an infected person.

Indirect Transmission

Most pathogens are transmitted indirectly through various means:

  • Airborne Droplets: Viruses causing colds, influenza, and Covid-19 are carried in the air in tiny droplets of moisture. Coughing or sneezing propels millions of viruses into the air (Figure 10.3). Breathing in these droplets can cause infection. Touching surfaces contaminated with these viruses and then touching your face can also lead to infection.
  • Contaminated Food: Bacteria like Salmonella can enter your alimentary canal through contaminated food, leading to food poisoning if a large number of bacteria are ingested. Washing fresh foods like fruits and vegetables in clean water is important. Cooking typically destroys bacteria, making recently cooked food generally safe. However, food kept warm for extended periods can allow bacteria to breed.
  • Contaminated Water: Many pathogens, including the virus that causes poliomyelitis and the bacterium that causes cholera, are transmitted in water. Swimming in or drinking contaminated water poses a risk of contracting these diseases (Figure 10.5). Pathogens can also be transferred from contaminated surfaces to your body when you touch your mouth or eat food with contaminated fingers.