8.2 Infectious Diseases

Introduction

Infectious diseases are illnesses caused by pathogens that enter the body and multiply. They can spread from person to person or from animals to people. Understanding infectious diseases is important for prevention, treatment, and control.

Definition

Infectious diseases are illnesses caused by pathogens that get into the body. The most common causes are viruses, bacteria, fungi, and parasites.

Infection: An infection occurs when a pathogen enters and multiplies inside a person's body, causing disease.

Pathogen: A pathogen is an agent that causes disease or infection.

Main Idea

• Infectious diseases can spread from person to person or from animals to people.
• An infection occurs when a pathogen enters and multiplies inside a person's body, causing disease.
• A pathogen is an agent that causes disease or infection.
• Types of pathogens include viruses, bacteria, fungi, protozoans, and parasitic worms.

They can spread in several different ways:

• Through skin contact
• The transfer of bodily fluids
• With faeces
• Ingesting contaminated food or water
• Inhaling airborne particles or droplets
• Touching an object that a person carrying the pathogen has also touched

Core Concept

Types of Pathogens and Their Diseases

Detailed Explanation

1. What are Infectious Diseases?

Infectious diseases can spread from person to person or from animals to people. An infection occurs when a pathogen enters and multiplies inside a person's body, causing disease.

Simple explanation: Infectious diseases are caused by germs (pathogens) that enter your body. They can spread from one person to another.

Why this is important:

• Infectious diseases are a major cause of illness and death worldwide.
• Understanding how they spread helps in prevention.
• Many infectious diseases can be treated or prevented.

2. What is a Pathogen?

A pathogen is an agent that causes disease or infection.

Types of pathogens include:

• Viruses
• Bacteria
• Fungi
• Protozoans
• Parasitic worms

Simple explanation: Pathogens are the germs that cause infectious diseases.

3. How Infectious Diseases Spread

They can spread in several different ways:

Simple explanation: Infectious diseases can spread through touch, body fluids, contaminated food or water, and air.

4. Types of Infectious Diseases

The types of infections are:

• Viral infection
• Bacterial infection
• Fungal infection
• Protozoan infection
• Parasitic infection

5. Types of Pathogens, Their Description, Examples, and Diseases

Simple explanation:

• Bacteria — single-celled organisms without a nucleus (e.g., Salmonella causes typhoid).
• Viruses — non-living particles that reproduce inside living cells (e.g., coronavirus causes COVID-19).
• Fungi — simple organisms that grow as threads (e.g., Dermatophytes cause ringworm).
• Protozoa — single-celled animal-like organisms (e.g., Plasmodium causes malaria).
• Parasites — multicellular organisms that live on or inside a host (e.g., Ascaris causes ascariasis).

Summary Table

Intermediate Level

Infectious diseases are illnesses caused by pathogens.

Modes of transmission:

• Skin contact
• Transfer of bodily fluids
• With faeces
• Ingesting contaminated food or water
• Inhaling airborne particles or droplets
• Touching contaminated objects

Types of infections:

• Viral infection
• Bacterial infection
• Fungal infection
• Protozoan infection
• Parasitic infection

Important Points

• - Infectious diseases can spread from person to person or from animals to people.
• - An infection occurs when a pathogen enters and multiplies inside a person's body, causing disease.
• - A pathogen is an agent that causes disease or infection.
• - Types of pathogens include viruses, bacteria, fungi, protozoans, and parasitic worms.
• - They can spread in several different ways: through skin contact, the transfer of bodily fluids, with faeces, ingesting contaminated food or water, inhaling airborne particles or droplets, touching an object that a person carrying the pathogen has also touched.
• - The types of infections are viral infection, bacterial infection, fungal infection, protozoan infection, and parasitic infection.
• - Bacteria are unicellular organisms without nucleus. Example: Salmonella typhii causes typhoid.
• - Viruses are non-living particles that reproduce by taking over living cells. Example: Coronavirus SARS-CoV-2 causes COVID-19.
• - Fungi are simple organisms that grow as thread-like filaments. Example: Dermatophytes cause ringworm.
• - Protozoa are unicellular organisms, animal-like Protista. Example: Plasmodium causes malaria.
• - Parasites are multicellular organisms that live as ectoparasites or endoparasites. Example: Ascaris causes ascariasis.

Remember

• - Infectious diseases = caused by pathogens
• - Pathogen = agent that causes disease
• - Types of pathogens = viruses, bacteria, fungi, protozoa, parasites
• - Bacteria = typhoid
• - Viruses = COVID-19
• - Fungi = ringworm
• - Protozoa = malaria
• - Parasites = ascariasis
• - Modes of transmission = skin contact, bodily fluids, faeces, contaminated food/water, airborne particles, contaminated objects

Common Mistakes

• 1. Confusing infectious and non-infectious diseases.
• - Infectious = caused by pathogens, can spread.
• - Non-infectious = not caused by pathogens, cannot spread.
• 2. Thinking all pathogens are bacteria.
• - Pathogens include viruses, bacteria, fungi, protozoa, and parasites.
• 3. Mixing up the diseases caused by different pathogens.
• - Bacteria = typhoid.
• - Viruses = COVID-19.
• - Fungi = ringworm.
• - Protozoa = malaria.
• - Parasites = ascariasis.
• 4. Forgetting the modes of transmission.
• - Skin contact, bodily fluids, faeces, contaminated food/water, airborne particles, contaminated objects.
• 5. Thinking infectious diseases cannot be prevented.
• - Infectious diseases can be prevented through hygiene, vaccination, and safe food/water.
• 6. Believing all infections are treatable with antibiotics.
• - Antibiotics only work against bacteria, not viruses, fungi, or parasites.
• 7. Confusing protozoa and parasites.
• - Protozoa = unicellular, animal-like Protista.
• - Parasites = multicellular organisms (e.g., worms).

Quick Revision

60-Second Revision

• Infectious diseases = caused by pathogens
• Pathogen = agent that causes disease
• Types of pathogens = viruses, bacteria, fungi, protozoa, parasites
• **Bacteria** = typhoid
• **Viruses** = COVID-19
• **Fungi** = ringworm
• **Protozoa** = malaria
• **Parasites** = ascariasis
• Modes of transmission = skin contact, bodily fluids, faeces, contaminated food/water, airborne particles, contaminated objects
• Warning: Do not confuse infectious (caused by pathogens, can spread) with non-infectious (not caused by pathogens, cannot spread) diseases.