Infectious disease has a biological cause

A disease is a disorder of body function that produces characteristic signs or symptoms. Some diseases are genetic; others come from environmental factors or are infectious.

An infectious disease is a disease caused by a pathogen that can enter a host, multiply or be replicated there, and spread directly or indirectly to another host. A pathogen is a disease-causing organism or infectious agent that causes harm after entering a host. In this topic, the term mainly refers to viruses, bacteria, fungi and protists that infect humans. Archaea are not known to cause human diseases.

For revision, it helps to sort human pathogens by type instead of trying to learn one long list. Bacteria, fungi, protists and viruses can all include human pathogens, but their structure and reproduction, or replication, differ.

Major human pathogen groups; archaea are not known to cause human disease.

Careful observation changed disease control

The nature of science point here is worth remembering: before microscopes and modern microbiology explained many infections, careful observation still saved lives. In Vienna, Ignaz Semmelweis connected childbed fever after childbirth with contamination carried on doctors’ hands and argued for handwashing. In London, John Snow mapped cholera cases and traced them to a contaminated water pump. These are not just historical anecdotes. They show how patterns in evidence can reveal causes, even before every mechanism is known.

Primary defence means preventing entry

A primary defence is a barrier or process that stops pathogens entering body tissues. Skin and mucous membranes count as primary defences because they act before a pathogen has established an infection.

The skin is an organ that forms a tough external covering over much of the body. As a physical barrier, its outer layers are hard for pathogens to get through, especially where cells are packed with the strong protein keratin. It also works as a chemical barrier: secretions such as sebum help maintain conditions that inhibit microbial growth, including a slightly acidic surface.

A mucous membrane is an epithelial lining that secretes mucus and protects body openings or internal passages exposed to the outside environment, such as airways and parts of the reproductive system. Mucus is a sticky secretion containing glycoproteins. It traps pathogens and particles so they can be swallowed, coughed out or otherwise removed. Mucus can also contain antimicrobial enzymes such as lysozyme.

You are not expected to draw or label skin diagrams for this syllabus point. The key idea is simple: intact surfaces are not passive wrapping; they are physical and chemical defences.

A clot is a rapid temporary seal

Blood clotting is a cascade process where soluble blood proteins are converted into an insoluble mesh that seals a wound. It reduces blood loss and closes a break in the skin barrier before pathogens can enter easily.

A platelet is a small cell fragment in blood that helps start clotting at damaged tissue. When a blood vessel is cut, platelets gather at the damaged area and release clotting factors. Those clotting factors begin a cascade: one activated factor activates the next, so the response is amplified quickly.

The cascade produces thrombin, an enzyme that converts soluble fibrinogen in the blood plasma into insoluble fibrin. Fibrin stretches as threads across the wound. The threads trap platelets and erythrocytes, which are red blood cells that transport oxygen, forming a clot. If exposed to air, the clot may dry into a scab while tissue repair continues underneath.

Two styles of immune response

The immune system is a body system made up of cells, tissues and molecules that defends the body against pathogens and abnormal cells. Its responses are usually split into innate immunity and adaptive immunity.

The innate immune system is a non-specific defence system that responds to broad categories of pathogen in much the same way throughout life. For this topic, the only innate immune cells you need to know are phagocytes. Skin and mucous membranes act as barriers; phagocytes are the specified cellular part of the innate response.

The adaptive immune system is a specific defence system that responds to particular antigens and improves after exposure because it keeps memory cells. This is the part responsible for highly targeted antibody responses, and it explains why the body can deal with a second encounter with the same pathogen more quickly.

This connects to the wider question of how animals protect themselves from threats. Humans use barriers, rapid non-specific cell responses and slower, specific immune responses. The key distinction is not “simple versus clever”; it is broad immediate defence versus specific memory-based defence.

Phagocytes move to infection and digest pathogens

A phagocyte is a white blood cell that protects the body by engulfing pathogens and digesting them inside the cell. When pathogens get through the skin or mucous membranes, phagocytes act as one of the next lines of defence.

Phagocytes can leave the blood and move into infected tissue. They move in an amoeboid way: the cell changes shape and pushes forward with extensions of cytoplasm. At the infection site, phagocytes recognize pathogens as foreign. They then engulf them by endocytosis, a process in which the plasma membrane surrounds material and brings it into the cell in a vesicle, and digest them using enzymes from lysosomes, organelles that contain hydrolytic digestive enzymes.

The end result is not glamorous, but it works. In an infected wound, large numbers of dead phagocytes, digested material and fluid may form pus.

Lymphocytes are the adaptive immune cells

A lymphocyte is a type of white blood cell involved in specific immune responses. They circulate in the blood, and many are found in lymph nodes — small organs of the lymphatic system where lymphocytes meet antigens and interact with one another.

A B-lymphocyte is a lymphocyte that, once activated, can produce one specific type of antibody. Students often miss the importance of that specificity: one person has a very large number of different B-lymphocytes, but each B-cell makes only one antibody type.

An antibody is a soluble protein made by B-lymphocytes that binds specifically to an antigen and helps eliminate the source of that antigen. Antibodies can make pathogens easier for phagocytes to engulf. Some also block viruses from attaching to host cells.

Before infection, there are only small numbers of B-cells for any one antigen, so lymphocytes have to cooperate. The matching B-cells must be selected, activated and multiplied before antibody concentration rises enough to control the infection.

Antigens are the labels the immune system reads

An antigen is a molecule that lymphocytes recognise as foreign or abnormal, triggering a specific immune response, including antibody production. In most pathogens, antigens are glycoproteins or other proteins, usually found on the pathogen’s outer surface.

Antibodies bind antigens only when the match is specific. The shape and chemical properties have to fit: the antibody’s binding region matches part of the antigen, rather like a receptor fitting a ligand. So one antibody type won’t bind equally well to every pathogen.

Antigens also occur away from pathogens. Erythrocyte antigens are surface molecules on red blood cells, and they can trigger antibody production if transfused into a person with an incompatible blood group. This is why blood typing and cross-matching are essential before transfusion.

B-cells need two matching signals

A helper T-lymphocyte is a lymphocyte that activates other immune cells after recognizing a specific antigen. B-cells don’t just meet an antigen and switch straight into full antibody production; the process is controlled so the right B-cells respond.

A B-cell needs two things before it becomes activated. First, it must interact directly with its specific antigen. Second, it must contact a helper T-cell that has also been activated by the same type of antigen. This double-check avoids wasting resources on irrelevant antibodies and lowers the chance of inappropriate immune responses.

The key word is “specific”. Antigen-specific B-cells and antigen-specific helper T-cells are involved. Only the rare cells with receptors matching that antigen are selected for the response.

Clonal expansion solves the numbers problem

A clone is a group of genetically identical cells produced from one original cell by mitosis. Early in an infection, the body may have only a few B-cells that can respond to a particular antigen. That’s not enough: a small number of cells cannot secrete enough antibody to clear a serious infection.

Once activated, the selected B-lymphocyte divides again and again by mitosis. The result is many B-cells with the same antibody specificity. Most of them then differentiate into plasma cells, B-cells specialized for rapid antibody secretion.

Plasma cells work like protein factories. They contain extensive rough endoplasmic reticulum and Golgi apparatus because antibodies are proteins that must be synthesized, processed and secreted. For the syllabus, the key idea is simple: activated B-cells multiply first, and the resulting plasma cells secrete large quantities of the same specific antibody.

Memory cells make the second response faster

Immunity is the body’s ability to eliminate an infectious disease. Antibodies that are already present can provide protection, but long-lasting immunity usually depends on memory cells.

A memory cell is a long-lived lymphocyte that remains after an immune response and can rapidly produce, or help produce, the same specific antibody if the antigen is encountered again. After an infection, most plasma cells die, so antibody levels fall. A smaller number of lymphocytes stay behind as memory cells.

If the same pathogen infects the body again, memory cells are activated quickly. The antibody response is faster and stronger, often eliminating the pathogen before symptoms become serious.

HIV transmission requires infected body fluids

HIV is the human immunodeficiency virus. It is transmitted in specific body fluids and, if untreated, can lead to AIDS. It does not spread through ordinary social contact, such as touching, sharing a classroom, or being near someone who is breathing normally.

HIV may be present in blood, semen, vaginal fluids, rectal secretions and breast milk. For transmission to happen, infected fluid must enter another person’s body, especially through mucous membranes, damaged tissue, direct blood contact or from mother to child.

Examples of transmission mechanisms include:

• sex without a condom, especially where small abrasions allow infected fluid to contact blood or mucous membranes;
• sharing hypodermic needles, because blood can be transferred directly;
• transfusion of infected blood or contaminated blood products, where screening is inadequate;
• childbirth and breastfeeding, where virus can pass from mother to child.

Public health control focuses on testing, safe sex, sterile needles, screened blood products and treatment that reduces viral load.

HIV damages the coordination of adaptive immunity

HIV infects and kills certain lymphocytes, especially helper T-lymphocytes. This matters because helper T-cells activate B-cells and coordinate antibody production. Their loss does not wipe out every immune cell at once, but it does make the body less able to mount effective specific responses.

An opportunistic infection is an infection caused by a pathogen that usually causes little disease in a person with a healthy immune system but can cause serious disease when immunity is weakened. As helper T-cell numbers fall, the body struggles to produce antibodies and fight these infections.

AIDS is a syndrome caused by advanced HIV infection in which severe immune deficiency allows a collection of opportunistic infections or related conditions to occur. A syndrome is a group of signs, symptoms or diseases that occur together and indicate a particular medical condition.

Antibiotics exploit bacterial differences

An antibiotic is a chemical that kills bacteria or inhibits their growth by blocking cellular processes. Here, the focus is on antibacterial antibiotics.

Bacterial cells are prokaryotic; human cells are eukaryotic. The most useful antibiotics hit processes or structures that bacteria have and human cells don’t, or ones that differ enough in bacteria that the drug damages the bacteria far more than the patient. Examples include bacterial cell wall formation, bacterial ribosome function, and bacterial DNA replication or transcription.

Comparison of antibacterial antibiotic targets in bacteria, human cells and viruses.

Antibiotics do not work against viruses for a straightforward reason: viruses do not have their own bacterial-type cells. A virus relies on host cell machinery to make viral components. It has no bacterial cell wall, no bacterial ribosomes and no independent metabolism for an antibiotic to target. If a drug blocked the host cell processes used by a virus, it could damage the patient’s own cells. Antibiotics therefore should not be used for viral infections unless there is a separate bacterial infection to treat.

Resistance evolves by selection

Antibiotic resistance is the ability of bacteria to survive exposure to an antibiotic that would normally kill them or stop their growth. A bacterial strain is a genetic variant within a bacterial species. If a pathogenic strain becomes resistant to several antibiotics, treatment becomes much harder.

Resistance evolves because bacterial populations vary. A few cells may already have a mutation or gene that lets them survive when an antibiotic is present. Once the antibiotic is used, susceptible bacteria are killed or inhibited, but resistant bacteria survive and reproduce. The antibiotic isn’t trying to create resistance; it acts as a selection pressure.

A multiresistant bacterium is a bacterium resistant to multiple antibiotics. Careful antibiotic use slows the emergence of multiresistant bacteria. That involves prescribing antibiotics only when there is good evidence of bacterial infection, choosing appropriate drugs and durations, preventing cross-infection in healthcare settings, and avoiding unnecessary antibiotic use in agriculture.

New techniques open new research routes

The nature of science point is that new techniques can create new avenues of research. By searching large chemical libraries, researchers can screen many molecules for antibacterial effects or for similarity to known useful structures. This doesn’t remove the need for careful use: a new antibiotic is valuable partly because resistance to it is not yet widespread.

Some pathogens cross species barriers

A zoonosis is an infectious disease that can pass naturally from another animal species to humans. Many pathogens infect only a narrow range of hosts, but zoonotic pathogens can cross between species, either directly or through vectors.

Zoonoses make up a major part of human infectious disease risk, and they don’t all spread in the same way. Close contact with livestock, bites from infected mammals, mosquito transmission, habitat disruption and wildlife contact can each create chances for transfer.

Examples of zoonoses showing source species, transfer route and main human impact.

Tuberculosis can be zoonotic when Mycobacterium bovis spreads from infected cattle to humans, for example through unpasteurized milk or droplets from infected animals. Rabies is caused by lyssaviruses and usually reaches humans through bites or scratches from infected mammals, especially dogs in many regions. Japanese encephalitis virus circulates in animals such as pigs or birds, then reaches humans through mosquito bites.

COVID-19 is also classed as a zoonotic disease because $SARS-CoV-2$ is thought to have transferred from another animal species to humans, probably with an origin in bats and possibly via an intermediate host. After that first transfer, human-to-human transmission led to profound global consequences.

Vaccines train immunity without causing the disease

A vaccine is a preparation containing antigens, or nucleic acids that code for antigens, that stimulates immunity to a specific pathogen without causing the disease. Immunization is the process of developing immunity, usually after vaccination or previous infection.

Some vaccines contain the antigen itself, such as a pathogen protein. Others contain DNA or RNA sequences that host cells use to make the antigen. In both cases, the immune system meets a recognizable molecule from the pathogen, without facing the full danger of the disease.

The vaccine triggers a primary immune response. Antigen-specific lymphocytes are activated, B-cells can form plasma cells, antibodies are produced, and memory cells may remain. If the real pathogen enters later, the secondary immune response is faster and stronger, making the pathogen more likely to be eliminated before serious disease develops.

That is why vaccination is not just “putting medicine in the body”. It uses the adaptive immune system’s own specificity and memory.

Immunity is shared protection at population level

Herd immunity means protection at the population level. It happens when enough people are immune to a pathogen that transmission is greatly slowed. It also shows interdependence clearly: my immune status can change the risk faced by someone else.

When many people are immune, an infected person is more likely to encounter immune individuals. That makes each chain of transmission more likely to break. As a result, epidemics are less likely, and vulnerable people who cannot be vaccinated, or who respond poorly to vaccines, get some protection.

A simple estimate for the herd immunity threshold is

If $R$ is higher, a higher proportion of the population must be immune.

Evidence, vaccines and uncertainty

Scientists publish research so other scientists can evaluate it. In an outbreak, media reports may come out before that evaluation has finished, so consumers need to separate early claims from well-supported conclusions. Vaccines are tested rigorously. The risk of side effects is very small, but it isn’t zero. Biology rarely gives absolute certainty, so public health decisions often use pragmatic truths: conclusions with enough evidence behind them to justify action, while safety and effectiveness continue to be monitored.

Evaluating pandemic data means weighing evidence

An evaluation is an evidence-based judgement: you weigh strengths, limitations and context before reaching a conclusion. COVID-19 data can include cases, deaths, testing rates, vaccination rates, population size, GDP per capita, age structure and dates of public health measures. Raw totals don’t usually tell you enough, because countries vary widely in population size and in the way they collected data.

Approximate cumulative COVID-19 data showing that raw totals and per-capita ranks can differ.

Useful derived values include cases per population, deaths per population and deaths as a percentage of confirmed cases. These give fairer comparisons than totals alone. They still have limits, though: under-testing, different reporting rules, timing of epidemic waves, access to healthcare, age distribution and vaccination coverage can all affect the numbers.

Percentages, percentage change and percentage difference

For a simple percentage,

Percentages have no units because the counts cancel.

For percentage change,

A positive value shows an increase; a negative value shows a decrease.

For percentage difference,

The trap is easy to miss: percentage difference changes depending on which number you choose as the reference.

Reporting research and diagnostic reliability

COVID-19 also shows how research results are reported. Scientific journals and the media may discuss findings while evaluation is still ongoing. Public advice can shift when better evidence builds up, as happened with several pandemic control measures. That kind of change is not automatically a failure; often, science is responding to stronger evidence.

The linking question about false-positive and false-negative diagnostic results fits here. A false-positive result is a test result that incorrectly indicates a condition is present. A false-negative result is a test result that incorrectly indicates a condition is absent. They are reduced by using validated tests with high specificity and sensitivity, including positive and negative controls, using appropriate sampling methods and timing, repeating or confirming important results, and interpreting test results alongside symptoms and exposure history.