Communicable Diseases¶

Part of Infection and Response.

Communicable disease is about transmission, defence and treatment. The important pattern is that pathogens enter a host, multiply, and trigger both natural and medical responses.

What You Need to Learn¶

Infectious disease depends on a pathogen reaching a host and overcoming its defences. Use the disease examples to connect symptoms, transmission and prevention, then explain how immunity, vaccination and medicines interrupt that process. Follow how a promising chemical becomes a tested medicine and why resistance and fair trials matter.

Pathogens and Disease Examples¶

• Pathogens include viruses, bacteria, fungi and protists. Different pathogens cause disease in different ways, but all disrupt normal body function.
• Useful examples include measles, HIV and tobacco mosaic virus for viral disease; salmonella and gonorrhoea for bacterial disease; rose black spot for fungal disease; and malaria for protist disease.
• Knowing the transmission route helps explain prevention. For example, sexually transmitted infections spread differently from diseases carried by water droplets or vectors.

Bacteria multiply in the body and may release toxins that damage tissues. Viruses reproduce inside host cells and can damage those cells. Pathogens spread through direct contact, contaminated water or air; hygiene, safe water, isolation where appropriate and control of vectors interrupt different routes.

Human Defence Systems and Vaccination¶

• The skin forms a barrier, the nose and trachea use mucus and cilia, and the stomach produces acid. These are general defences that act before the specific immune response starts.
• White blood cells defend the body by phagocytosis, by making antibodies that bind to pathogens, and by producing antitoxins that neutralise toxins.
• Vaccination exposes the immune system to safe forms of antigens so that memory cells are produced before real infection occurs.

Skin provides a physical barrier. Nose hairs and mucus trap particles; cilia in the trachea and bronchi move mucus towards the throat for swallowing. Stomach acid kills many swallowed pathogens. If these barriers fail, phagocytes engulf and digest pathogens, while antibody-producing white cells recognise particular antigens and antitoxins neutralise bacterial toxins.

A vaccine supplies harmless antigen material, such as an inactive pathogen, to stimulate a specific immune response. On later exposure, memory cells enable rapid production of the matching antibodies. When many people are immune, the pathogen has fewer susceptible hosts, so transmission is reduced and some people who cannot be vaccinated gain protection.

Antibiotics, Painkillers and Drug Development¶

• Antibiotics kill bacteria or stop them reproducing by targeting bacterial structures or processes. Viruses lack these targets and reproduce using host cells; killing them without harming those cells is difficult.
• Painkillers reduce symptoms without killing pathogens, so feeling better does not necessarily mean the infection has been removed.
• New drugs are tested in the laboratory, then in clinical trials, so that toxicity, effectiveness and optimum dose can be checked carefully.

The antibiotic must match the bacterium: resistant bacteria survive drugs that would previously have worked, making treatment harder. This is why antibiotic resistance threatens the gains made in treating bacterial disease.

Digitalis originated from foxgloves, aspirin from willow-derived chemicals, and penicillin from mould observed by Alexander Fleming in 1928. Modern chemists often synthesise medicines, but natural chemicals can still provide the starting idea.

Preclinical tests use cells, tissues and animals to investigate toxicity and whether a drug works. Clinical trials then involve volunteers and patients, beginning with low doses and progressing to comparisons that establish an effective, tolerable dose. A placebo resembles the treatment but lacks its active drug, providing a comparison for effects unrelated to the drug. In a double-blind trial, participants and the researchers assessing them do not know who received the drug, reducing bias.

Common Confusions¶

• Antibiotics vs. antivirals: Antibiotics kill bacteria; antivirals inhibit viral replication. Antibiotics do not work against viruses because viruses lack the bacterial targets on which these drugs act.
• Antitoxins vs. antibodies: Antitoxins are specific proteins that neutralise bacterial toxins; antibodies are proteins that bind to antigens on pathogens. Antitoxins are one type of antibody.
• Vaccination vs. immunity: Vaccination introduces antigens safely; immunity is the resulting protection. Vaccination creates memory cells, so the immune response is faster if real infection occurs.
• Painkillers vs. antibiotics: Painkillers mask symptoms (you feel better) but don't kill pathogens (infection continues). Antibiotics kill pathogens. Symptom relief does not show that the pathogen has been eliminated.

Key Terms¶

• Pathogen: a microorganism or agent that causes disease.
• Communicable disease: a disease that can be passed from one organism to another.
• Vaccination: introducing antigens safely so the immune system can prepare memory cells.
• Antibody: a protein produced by white blood cells that binds to a specific antigen.
• Antitoxin: a protein that neutralises a specific bacterial toxin.
• Phagocytosis: the process by which white blood cells engulf and digest pathogens.
• Antibiotic: a drug that kills bacteria or stops them reproducing.
• Antiviral: a drug that inhibits viral replication.
• Pathogenic: capable of causing disease.
• Clinical trial: careful testing of a treatment on people to assess safety and effectiveness.

Connected Pages¶

• Cell Structure
• Monoclonal Antibodies
• Plant Disease
• Biology Key Terms Glossary