We are surrounded by microorganisms. They are in the air, water, soil, food and even on and inside our bodies. Most do not make us ill. Many are harmless, and some are useful. Problems arise when a disease-causing organism or infectious agent enters the body, survives and interferes with normal body functions.
These disease-causing agents are called pathogens. Understanding them is not simply a matter of memorising a list of diseases. The important questions are: What type of pathogen causes the disease? How is it transmitted? How does our immune system respond? And why are vaccines and antibiotics useful in very different ways?
Try these five questions before reading the chapter. They check whether you can already distinguish infectious disease, viruses, bacteria, vaccines and antibiotics.
- 5 diagnostic questions
- Core pathogen and immunity concepts
- Instant explanation for every answer option
Infectious and non-infectious diseases are fundamentally different
A useful starting point is to separate diseases into infectious and non-infectious diseases.
An infectious disease is caused by a pathogen and can be transmitted from one host to another, either directly or indirectly. Influenza and pneumococcal disease are examples.
A non-infectious disease is not caused by a transmissible pathogen and therefore cannot simply be passed from one person to another. Such diseases may involve genetic, environmental or lifestyle factors. Examples include diabetes, coronary heart disease and asthma.
| Feature | Infectious disease | Non-infectious disease |
|---|---|---|
| Caused by a pathogen? | Yes | No |
| Can be transmitted between hosts? | Yes, directly or indirectly | No |
| Examples | Influenza, pneumococcal disease | Diabetes, coronary heart disease, asthma |
Core idea: “infectious” describes the cause and transmission of a disease. It does not mean simply that the disease is severe, produces a fever or requires medicine.
Pathogens cause infectious disease
A pathogen is a disease-causing organism or infectious agent. Pathogens can enter the body, multiply and interfere with normal cell, tissue or organ function.
At this level, two especially important groups are viruses and bacteria. They are sometimes casually grouped together as “germs”, but biologically they are very different.
Do not write that “all microorganisms are harmful”. Many microorganisms are harmless, and some are beneficial. The term pathogen refers specifically to one that causes disease.
How infectious diseases are transmitted
For an infectious disease to spread, a pathogen must move from an infected source to another susceptible host. Different pathogens use different routes of transmission.
| Route | How transmission may occur | Example situation |
|---|---|---|
| Respiratory droplets or particles | Pathogens leave the respiratory tract when an infected person coughs, sneezes, talks or breathes | Influenza |
| Body fluids | Infected blood or other body fluids pass from one person to another | Transmission depends on the particular pathogen |
| Contaminated food | Pathogens are swallowed in food that has been contaminated | Food-borne infections |
| Contaminated water | Pathogens enter the digestive system when unsafe water is consumed | Water-borne infections |
| Direct contact with respiratory secretions | Pathogens pass through close contact with secretions from the nose or throat | Pneumococcal disease |
Breaking the route of transmission helps reduce the spread of disease. Depending on the pathogen, useful measures may include hand hygiene, safe food preparation, clean water, good ventilation, reducing close contact when ill and vaccination.
Think of transmission as a chain: infected source → route of transmission → susceptible host. Preventive measures work by breaking one or more links in that chain.
Viruses depend on living host cells
A virus is much simpler than a cell. A typical virus contains genetic material, which may be DNA or RNA, enclosed within a protein coat. Some viruses also possess an outer envelope.
Crucially, a virus does not contain all the machinery required to reproduce independently. It must enter a suitable living host cell.
Once inside, viral genetic material directs the host cell to produce viral components. These components are assembled into new virus particles, which are then released and can infect additional cells.
Different viruses leave their host cells in different ways. Some cause the cell to break open, while others leave without immediately destroying it. The examination point to remember is the same: viral reproduction depends on a living host cell.
Virus reproduction: virus enters suitable host cell → viral genetic material uses host-cell machinery → viral components are produced → new virus particles assemble → viruses are released.
Bacteria are living single-celled organisms
Bacteria are very different from viruses. A bacterium is a complete living cell, although it is structurally simpler than a typical plant or animal cell.
A bacterial cell contains cytoplasm, a cell surface membrane, ribosomes and genetic material. It normally has a cell wall. Its main DNA is not enclosed inside a nucleus.
Under suitable conditions, bacteria can reproduce independently, commonly by dividing into two. They therefore do not need to enter another cell simply in order to reproduce.
Importantly, not all bacteria are pathogenic. Many bacterial species do not cause disease, and some form part of the normal microorganisms living on or inside the human body.
| Feature | Virus | Bacterium |
|---|---|---|
| Cellular organisation | Not a cell | Single-celled organism |
| Genetic material | DNA or RNA | DNA |
| Protein coat | Present | Not the defining outer structure |
| Cell membrane and cytoplasm | Not organised as a complete cell | Present |
| Nucleus | Absent | Absent; DNA is not enclosed in a nucleus |
| Independent reproduction | No — requires a living host cell | Yes, under suitable conditions |
| Antibiotics | Ineffective against the virus itself | Some bacterial infections can be treated with suitable antibiotics |
Influenza and pneumococcal disease show why the type of pathogen matters
Influenza and pneumococcal disease can both affect the respiratory system and may share symptoms such as fever and cough. However, they are caused by different types of pathogens. That difference has important consequences for prevention and treatment.
| Feature | Influenza | Pneumococcal disease |
|---|---|---|
| Cause | Influenza virus | Streptococcus pneumoniae, a bacterium also called pneumococcus |
| Type of pathogen | Virus | Bacterium |
| Possible symptoms | Fever, cough, sore throat, headache, tiredness and muscle aches | May cause pneumonia with fever, cough, chest pain and difficulty breathing |
| Transmission | Mainly through respiratory droplets or particles and contaminated respiratory secretions | Through respiratory droplets or close contact with respiratory secretions |
| Vaccination | Influenza vaccines can reduce the risk of influenza | Pneumococcal vaccines can protect against important pneumococcal infections |
| Antibiotics | Do not kill the influenza virus | Suitable antibiotics may treat susceptible bacterial infections |
Exam link: influenza = virus; pneumococcal disease = bacterium. This difference explains why an antibiotic may be useful against a susceptible pneumococcal infection but will not kill the influenza virus.
Vaccination prepares the immune system before infection
A vaccine exposes the immune system to antigens associated with a particular pathogen without causing the full disease that the pathogen normally produces.
The exact contents of vaccines differ, but the underlying biological principle is the same. The immune system recognises the relevant antigen as foreign. Specific lymphocytes respond, antibodies may be produced, and memory cells are formed.
If the vaccinated person later encounters the actual pathogen, these memory cells allow a faster and stronger secondary immune response. The pathogen may therefore be destroyed before it causes severe disease.
Vaccination sequence: vaccine introduces relevant antigen → specific lymphocytes respond → antibodies are produced and memory cells form → later exposure to the same antigen produces a faster immune response.
Vaccination is therefore mainly a preventive strategy. It prepares the immune system in advance. It should not be confused with an antibiotic, which is a medicine used against susceptible bacteria.
Vaccination does not mean that infection becomes impossible in every vaccinated person. Protection can vary between vaccines and individuals. The central biological idea is that immune memory increases the body's ability to respond rapidly when the pathogen is encountered again.
Antibiotics target bacteria, not viruses
Antibiotics are medicines used to treat susceptible bacterial infections. Different antibiotics act in different ways. For example, some interfere with bacterial cell-wall formation, while others interfere with bacterial protein synthesis or other essential bacterial processes.
These drugs can work because bacteria possess cellular structures and biochemical processes that can be targeted.
Viruses present a completely different problem. A virus does not have the same cellular machinery as a bacterium and reproduces by using processes inside a host cell. The bacterial targets of an antibiotic are therefore absent.
Do not memorise simply “antibiotics kill germs”.
A better statement is: antibiotics act against susceptible bacteria, but they do not kill viruses.
This is why taking an antibiotic for an uncomplicated viral infection such as influenza does not eliminate the virus. Antibiotics may sometimes be prescribed if a bacterial infection is also present, but that is a separate bacterial problem.
Antibiotic resistance develops by natural selection
Antibiotics have saved many lives, but bacterial populations can evolve antibiotic resistance. This means that an antibiotic which once killed or inhibited a bacterium may become much less effective against a resistant strain.
Resistance does not appear because bacteria deliberately “try” to become resistant. Instead, bacterial populations contain genetic variation. A mutation or an acquired resistance gene may make some bacteria less affected by a particular antibiotic.
Natural selection of antibiotic resistance:
1. A bacterial population contains genetic variation.
2. Some bacteria may possess resistance to an antibiotic.
3. Antibiotic treatment kills or inhibits susceptible bacteria.
4. Resistant bacteria are more likely to survive.
5. The survivors reproduce and pass resistance genes to later generations.
6. Resistant bacteria become a larger proportion of the population.
Antibiotic use therefore creates a selection pressure. The antibiotic does not intentionally create stronger bacteria. Instead, it changes which bacteria are most likely to survive and reproduce.
| Practice | Why it matters |
|---|---|
| Use antibiotics only when medically appropriate | Unnecessary exposure increases selection for resistant bacteria without treating viral infections. |
| Take antibiotics exactly as prescribed | Incorrect use may reduce treatment effectiveness and contribute to selection of resistant bacteria. |
| Complete the prescribed treatment unless a healthcare professional advises otherwise | Changing treatment duration independently may leave an infection inadequately treated. |
| Do not share leftover antibiotics | Different infections may require different treatment, and unnecessary use promotes resistance. |
| Do not use antibiotics to treat viral infections | Antibiotics do not kill viruses, while unnecessary antibiotic exposure still selects among bacteria. |
The person does not become resistant to an antibiotic. The bacteria become resistant.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Calling every microorganism a pathogen | Many microorganisms are harmless or beneficial. | Use pathogen specifically for a disease-causing organism or infectious agent. |
| Saying viruses are tiny bacterial cells | Viruses are not cells and are fundamentally different from bacteria. | State that viruses contain genetic material in a protein coat and reproduce only inside living host cells. |
| Saying all bacteria cause disease | Only some bacterial species are pathogenic. | Distinguish pathogenic bacteria from harmless or beneficial bacteria. |
| Saying antibiotics kill viruses | Viruses lack the bacterial structures and metabolic processes targeted by antibiotics. | State that antibiotics act against susceptible bacteria, not viruses. |
| Saying vaccination immediately kills a pathogen | A vaccine mainly prepares the immune system before later exposure. | Explain antigen recognition, antibody production, memory cells and a faster secondary response. |
| Saying antibiotics make bacteria resistant because the bacteria “get used to” the medicine | This misses the role of genetic variation and natural selection. | Explain that resistant variants survive antibiotic treatment and reproduce. |
| Saying a patient's body becomes antibiotic-resistant | Resistance is a property of the bacteria. | Write that antibiotic-resistant bacteria survive and spread. |
| Confusing influenza with pneumococcal disease | They are caused by different types of pathogens. | Remember: influenza → virus; pneumococcal disease → bacterium. |
What you should be able to do
Distinguish infectious and non-infectious disease
Explain that infectious diseases are caused by transmissible pathogens, whereas non-infectious diseases are not.
Define a pathogen
Describe a pathogen as a disease-causing organism or infectious agent without assuming that every microorganism is harmful.
Compare viruses and bacteria
Relate their different structures to their different methods of reproduction and response to antibiotics.
Explain disease transmission
Describe how pathogens may spread through respiratory secretions, body fluids, contaminated food or contaminated water.
Compare influenza and pneumococcal disease
Identify influenza as viral and pneumococcal disease as bacterial, and relate this distinction to prevention and treatment.
Explain how vaccination provides protection
Link antigen exposure to lymphocyte activation, antibody production, memory cells and a faster response during later infection.
Explain antibiotic action
State why antibiotics can act against susceptible bacteria but are ineffective against viruses.
Explain antibiotic resistance by natural selection
Describe variation, survival of resistant bacteria, reproduction and the increasing frequency of resistance in a population.
These ten questions test whether you can apply the ideas of transmission, vaccination, antibiotics and natural selection rather than simply recall definitions.
- 10 mastery questions
- Application and exam-style reasoning
- Instant explanation for every answer option
Frequently asked questions
Organisms affecting Human Health: questions students often ask
Once you can distinguish viruses from bacteria, most remaining confusion comes from three ideas: what counts as a pathogen, why vaccines and antibiotics are different, and how antibiotic resistance actually develops.
Are all microorganisms pathogens?
No. A pathogen is specifically an organism or infectious agent that causes disease. Many microorganisms are harmless, while others may be beneficial. For example, many bacteria normally live in and on the human body without causing disease.
Is a virus a living organism?
Viruses have some features associated with living systems, such as genetic material and the ability to evolve, but they do not have a cellular structure and cannot reproduce independently. They must use a living host cell. For O-Level Biology, the most important point is therefore that a virus is not a cell and reproduces only inside a suitable living host cell.
Why do antibiotics kill bacteria but not viruses?
Antibiotics act on particular bacterial structures or biochemical processes, such as cell-wall formation or bacterial protein synthesis. Viruses do not possess the same cellular machinery. Instead, they use machinery inside the host cell to reproduce, so ordinary antibiotics do not kill them.
Can vaccines protect us from both viral and bacterial diseases?
Yes. Vaccination works by training the immune system to recognise particular antigens, so vaccines can be developed against either viral or bacterial pathogens. Influenza vaccines target a viral disease, whereas pneumococcal vaccines protect against disease caused by bacteria.
Does vaccination mean I can never become infected?
Not necessarily. Vaccine effectiveness varies, and protection may not be complete in every person. The important biological effect is that vaccination creates immune memory, allowing the immune system to respond more rapidly and effectively during later exposure. This can greatly reduce the risk or severity of disease.
Does the human body become resistant to an antibiotic?
No. When we talk about antibiotic resistance, it is the bacteria that are resistant. Resistant bacteria survive exposure to an antibiotic better than susceptible bacteria and can then reproduce and spread their resistance genes.
Does an antibiotic cause bacteria to mutate so that they become resistant?
That is not the main explanation. Genetic variation may already exist in the bacterial population, or resistance genes may be acquired. When an antibiotic is applied, susceptible bacteria are killed or inhibited more readily while resistant bacteria survive. The antibiotic therefore acts mainly as a selection pressure.
Why is using antibiotics for influenza a problem?
Influenza is caused by a virus, so an antibiotic does not kill the influenza virus. Unnecessary antibiotic use can still expose bacteria in the body to the drug, creating additional selection pressure that favours resistant bacteria.
What is the easiest way to remember influenza and pneumococcal disease?
Focus on the pathogen rather than memorising a long list of symptoms: influenza is caused by a virus, whereas pneumococcal disease is caused by the bacterium Streptococcus pneumoniae. Once that distinction is clear, the difference in antibiotic treatment also makes sense.
The central idea of this chapter is that the identity of the pathogen explains much of what follows. Viruses and bacteria have different structures and reproduce differently, so they cannot always be controlled in the same way. Vaccination prepares the immune system before infection, antibiotics act only against susceptible bacteria, and antibiotic resistance develops when natural selection favours resistant bacterial variants. Once those links are clear, the topic becomes a connected story rather than a collection of facts to memorise.