A pathogen is a microorganism that causes disease. Communicable diseases are caused by pathogens that can be transmitted from one organism to another, unlike non-communicable diseases such as cancer or diabetes. The four types of pathogen are bacteria, viruses, fungi, and protists — each with a distinct structure, mechanism of harm, and treatment approach.
What is a pathogen and what makes a disease communicable?
A pathogen is any microorganism capable of causing disease in a host. A communicable disease (also called an infectious disease) is one that can spread between individuals — directly by contact, indirectly through the air, water, food, or via a vector such as a mosquito.
Non-communicable diseases — including cardiovascular disease, type 2 diabetes, and most cancers — are not caused by pathogens and cannot be passed between individuals; they arise from genetic, lifestyle, or environmental factors.
What are the four types of pathogen?
| Pathogen type | Structure | How they cause harm | Example disease | Treatment |
|---|---|---|---|---|
| Bacteria | Living single-celled organisms (prokaryotes); no nucleus | Reproduce rapidly inside the host; release toxic waste products (toxins) | Tuberculosis (TB), Salmonella food poisoning, cholera | Antibiotics (kill or inhibit bacteria) |
| Viruses | Non-cellular; a protein coat (capsid) surrounding genetic material | Invade host cells, hijack the cell's machinery to replicate, then burst the cell (lysis) | Influenza, HIV, measles, COVID-19 | Antiviral drugs; vaccines prevent infection |
| Fungi | Eukaryotic; some produce spores | Grow on or in the body; produce toxins; destroy tissue | Athlete's foot, ring worm, rose black spot in plants | Antifungal medicines |
| Protists | Eukaryotic single-celled organisms; often parasitic | Live and reproduce inside host cells or tissues, damaging them | Malaria (caused by Plasmodium) | Antimalarial drugs; preventing vector bites |
Key distinction — bacteria vs viruses: Bacteria are living cells that can be killed by antibiotics. Viruses are not alive in the conventional sense and are unaffected by antibiotics; antiviral drugs or vaccines are required instead. Using antibiotics for a viral infection is ineffective and contributes to antibiotic resistance.
How do pathogens spread?
Transmission routes vary by pathogen and determine control measures:
- Droplets in air — influenza and COVID-19 spread when infected people cough or sneeze; droplets are inhaled by others. Wearing masks and improving ventilation reduce spread.
- Direct contact — athlete's foot and other fungal infections pass by skin-to-skin contact or touching infected surfaces.
- Contaminated water or food — cholera bacteria (Vibrio cholerae) spread via water contaminated with faecal matter; Salmonella spreads via undercooked poultry. Safe water, sewage treatment, and food hygiene are the key controls.
- Vectors — malaria is transmitted by the female Anopheles mosquito, which carries Plasmodium protists in its saliva. Controlling mosquito breeding sites (removing standing water) and using bed nets reduce malaria transmission.
- Sexual contact / bodily fluids — HIV spreads through blood, breast milk, and sexual contact; barrier contraception and not sharing needles reduce risk.
How does the body defend itself against pathogens?
The body has three layers of defence:
1. Physical and chemical barriers (first line of defence)
- The skin forms an impermeable barrier when unbroken.
- Mucus produced in the airways traps pathogens; cilia on airway cells sweep mucus and trapped particles up to the throat to be swallowed.
- Stomach acid (pH ~2) kills most pathogens swallowed with food or trapped mucus.
2. Non-specific immune response (second line of defence)
- Phagocytes (a type of white blood cell) engulf and digest pathogens by phagocytosis — they do not recognise specific pathogens but respond to any foreign material.
- Inflammation and fever are part of this response.
3. Specific immune response (third line of defence)
- Lymphocytes (another type of white blood cell) produce specific antibodies that match the antigens (proteins) on the surface of a particular pathogen.
- Antibodies bind to antigens, neutralising the pathogen or marking it for destruction by phagocytes.
- After infection, memory lymphocytes remain, allowing a much faster, larger antibody response if the same pathogen is encountered again — this is immunological memory and the basis of vaccination.
What is the difference between vaccination and treatment?
Vaccination is a preventive measure — a weakened, dead, or partial form of a pathogen is introduced into the body, stimulating the immune system to produce antibodies and memory cells without causing disease. If the real pathogen later enters the body, the immune response is so rapid that disease is prevented or greatly reduced.
Treatment addresses disease after infection: antibiotics treat bacterial infections; antiviral drugs reduce the severity and duration of viral infections; antifungals treat fungal disease; antimalarials treat protist infections.
Frequently asked questions
What is the difference between a communicable and a non-communicable disease?
A communicable disease is caused by a pathogen (bacteria, virus, fungus, or protist) and can spread from one host to another through direct contact, the air, contaminated food or water, or a vector. A non-communicable disease cannot be passed between individuals and is caused by genetic, lifestyle, or environmental factors — examples include coronary heart disease, type 2 diabetes, and lung cancer.
Why do antibiotics not work against viruses?
Antibiotics target specific structures or processes found in bacterial cells — for example, the bacterial cell wall or protein synthesis machinery. Viruses do not have cell walls; they are not cells at all. Viruses replicate inside host cells using the host's own machinery. Antibiotics have nothing to target in a virus and so have no effect. Taking antibiotics for a cold or flu is ineffective and unnecessarily contributes to the development of antibiotic-resistant bacteria.
How does malaria differ from most other communicable diseases?
Malaria is caused by a protist (Plasmodium) rather than a bacterium or virus, and is transmitted exclusively by the bite of the female Anopheles mosquito, which acts as a vector — carrying the pathogen from one host to another without itself being harmed by it. Inside the human body, Plasmodium invades red blood cells and liver cells, destroying them and releasing toxins that cause the characteristic cycles of fever. There is no vaccine in routine use (as of 2026), so prevention focuses on controlling mosquito populations and protecting individuals from bites.
What is immunological memory and how does it make vaccines work?
When the immune system first encounters a pathogen (or a vaccine), lymphocytes produce antibodies and some of these lymphocytes remain as memory cells for many years or decades. If the same pathogen is encountered again, these memory cells respond almost immediately — producing large quantities of antibodies within hours rather than days. This rapid response usually prevents symptoms from developing. Vaccines exploit this by exposing the immune system to a harmless version of the pathogen so that memory cells are ready before real infection occurs.
For Socratic GCSE biology with Professor Darwin — tracing how a pathogen moves from molecule to cell to organism to epidemic — visit aitutors.me.