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, and others 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 means viruses, bacteria, fungi and protists that infect humans. Archaea are not known to cause human diseases.

When revising this first idea, group human pathogens by type rather than trying to memorise one long list. Bacteria, fungi, protists and viruses can all include human pathogens, but their structures differ, as do the ways they reproduce or are replicated.

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 keeping clear: 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, then promoted handwashing. In London, John Snow mapped cholera cases and traced them to a contaminated water pump. These aren’t just historical anecdotes. They show that 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 work before a pathogen has established an infection.

The skin is an organ that forms a tough external covering over much of the body. Its outer layers make a physical barrier, since pathogens struggle to pass through them, especially where cells are packed with the strong protein keratin. Skin also acts 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. Focus on the principle: 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 in which soluble blood proteins are converted into an insoluble mesh that seals a wound. This matters for two reasons: it reduces blood loss, and it closes a break in the skin barrier before pathogens can enter easily.

A platelet is a small cell fragment in blood that helps initiate clotting at damaged tissue. When a blood vessel is cut, platelets collect at the damaged area and release clotting factors. These clotting factors start a cascade, meaning one activated factor activates the next, so the response is rapidly amplified.

The cascade leads to production of thrombin, an enzyme that converts soluble fibrinogen in the blood plasma into insoluble fibrin. Fibrin forms threads across the wound. These 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 of cells, tissues and molecules that defends against pathogens and abnormal cells. Its responses are usually grouped as innate or 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 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 retains memory cells. This is the part of immunity that produces highly targeted antibody responses, which is why a second encounter with the same pathogen can be dealt with faster.

This connects with 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. If pathogens get through the skin or mucous membranes, phagocytes act as one of the next lines of defence.

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

The end result isn’t 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. Many lymphocytes travel in the blood, while others sit in lymph nodes, small organs of the lymphatic system where lymphocytes meet antigens and interact with each other.

A B-lymphocyte is a lymphocyte that, once activated, can produce one specific type of antibody. Students often miss the specificity here: 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. It binds specifically to an antigen and helps eliminate the source of that antigen. Antibodies can make pathogens easier for phagocytes to engulf, and some can block viruses from attaching to host cells.

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

Antigens are the labels the immune system reads

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

Antibodies bind antigens with a specific fit. The shape and chemical properties have to match: the antibody’s binding region fits part of the antigen rather like a receptor fits 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 that can trigger antibody production if they are transfused into a person with an incompatible blood group. That’s 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 response is controlled, so the right B-cells are the ones that respond.

A B-cell needs two conditions before it can be 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”. Some B-cells are antigen-specific, and so are some helper T-cells. Only the rare cells with receptors that match 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 isn’t 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. It makes 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 have extensive rough endoplasmic reticulum and Golgi apparatus because antibodies are proteins, so they must be synthesized, processed and secreted. The key syllabus idea is simple: activated B-cells multiply first; the resulting plasma cells then secrete large quantities of the same specific antibody.

Memory cells make the second response faster

Immunity is the ability to eliminate an infectious disease from the body. Antibodies already present may provide it, but long-lasting immunity usually relies 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 and antibody levels fall. A smaller number of lymphocytes remain as memory cells.

If the same pathogen infects the body a second time, memory cells activate 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 can lead to AIDS if untreated. HIV is not spread by 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. Transmission happens when infected fluid enters 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 transfer 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 help coordinate antibody production. When they are lost, not every immune cell disappears at once, but the immune system becomes much less able to produce 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 has more trouble producing antibodies and fighting 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 slows their growth by blocking cellular processes. For this syllabus point, 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 do not, or ones that are different enough in bacteria for the drug to harm 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 simple reason: viruses do not have their own bacterial-type cells. A virus uses host cell machinery to make viral components. It has no bacterial cell wall, no bacterial ribosomes and no independent metabolism to target. If a drug blocked the host cell processes used by a virus, it could damage the patient’s own cells. So antibiotics should not be used for viral infections unless there is a separate bacterial infection to treat.

Resistance evolves by selection

Antibiotic resistance means bacteria can 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. When a pathogenic strain becomes resistant to several antibiotics, treatment gets much harder.

Resistance evolves because bacterial populations are not all the same. Some cells may already have a mutation or gene that lets them survive when an antibiotic is present. When the antibiotic is used, susceptible bacteria are killed or inhibited. Resistant bacteria survive and reproduce. The antibiotic doesn’t try 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 means 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 open new avenues of research. By searching large chemical libraries, researchers can screen many molecules for antibacterial effects or for similarity to known useful structures. Even so, careful use still matters: 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 be transmitted naturally from another animal species to humans. Many pathogens infect only a narrow host range, 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 all give pathogens more chances to transfer.

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

Tuberculosis can be zoonotic when Mycobacterium bovis passes 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 produced 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, for example 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’s 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 across a population when enough individuals are immune to a pathogen that transmission is greatly impeded. It’s a clear example of interdependence: one person’s immune status can change the risk faced by someone else.

When many people are immune, an infected person is more likely to meet immune individuals. Chains of transmission then have more chances to stop. This helps prevent epidemics, and it protects vulnerable people who cannot be vaccinated or who respond poorly to vaccines.

A simple estimate for the herd immunity threshold is H = (1 − 1/R) × 100%, where H is the estimated percentage of the population that must be immune (%) and R is the average number of secondary infections caused by one infected person in a fully susceptible population (unitless). As R rises, so does the proportion that must be immune.

Evidence, vaccines and uncertainty

Scientists publish research so other scientists can evaluate it. During outbreaks, media reports may appear before that evaluation has finished, so consumers need to separate early claims from well-supported conclusions. Vaccines are rigorously tested; the risk of side effects is very small but not zero. Biology rarely gives absolute certainty, so public health decisions often use pragmatic truths: conclusions strongly supported enough by evidence to act on, while continuing to monitor safety and effectiveness.