5.1.1 Communicable and non-communicable diseases

Health Includes Physical, Mental and Social Wellbeing

1. Health is a state of physical, mental and social wellbeing, not only the absence of disease.
2. Physical wellbeing means the body and its organs can function effectively.
3. Mental wellbeing means you can think, feel and cope with ordinary demands without persistent distress.
4. Social wellbeing means you can form supportive relationships and take part in your community.
5. Health can be affected by diet, stress, living conditions, income and access to healthcare because these change exposure to disease and the body's ability to cope with it.
6. Health is affected by interactions between physical, mental and social factors, so a change in one area can alter the others.
7. Long-term physical illness may limit movement and social contact, which can worsen mental wellbeing.
8. Poor mental health may reduce appetite, sleep and willingness to seek treatment, which can worsen physical health.
9. Living conditions affect exposure to pathogens, while income and healthcare access affect prevention, diagnosis and treatment.

Diseases Are Communicable or Non-communicable

1. A communicable disease is an illness caused by a pathogen that can pass between organisms.
   1. Examples include influenza caused by a virus, salmonellosis caused by bacteria, and malaria caused by a protist.
2. A non-communicable disease is an illness that is not caused by a pathogen and cannot pass between organisms.
   1. Examples include cardiovascular disease, type 2 diabetes and cancer.
3. Non-communicable diseases may be linked to lifestyle, inherited alleles, environmental exposure or nutrient deficiency.
4. When comparing diseases, identify both cause and transmissibility because either feature alone gives an incomplete distinction.

5.1.2 Disease susceptibility interactions

One Disease Can Increase the Risk of Another

1. The immune system protects the body by recognising and destroying pathogens and infected cells.
2. Susceptibility is the likelihood that a person will develop a disease after exposure to its cause.
3. A first disease can damage a defence barrier, weaken immune responses or alter body tissues, which increases susceptibility to a second disease.
4. A causal explanation must identify the first disease, the biological change it causes and why that change increases the second disease.

HIV Weakens Immune Defence

1. HIV infects and destroys particular white blood cells that coordinate immune responses.
2. As the number of these cells falls, the body produces a weaker response to pathogens.
3. Pathogens that would normally be controlled can reproduce and cause opportunistic infections.
4. Untreated HIV infection can progress to AIDS, when immune function is severely damaged.
5. HIV binds to specific white blood cells and uses them to reproduce, progressively reducing the cells available to coordinate immune responses.
6. With fewer effective lymphocytes, antibody production and activation of other immune cells become less effective.
7. Opportunistic pathogens then reproduce to damaging numbers even when they would usually be controlled.

Barrier Damage Creates Entry Routes

1. Intact skin forms a physical barrier that prevents pathogens entering body tissues.
2. A burn, wound or skin disease breaks this barrier and exposes tissue beneath it.
3. Bacteria can then enter, reproduce and cause an infection.

Infections Can Trigger Other Diseases

1. Some viruses alter the control of cell division, which can increase the risk of certain cancers.
2. For example, persistent infection with some strains of human papillomavirus increases the risk of cervical cancer.
3. A severe physical disease can also reduce activity, disrupt sleep and cause long-term stress, which can worsen mental health.
4. Some infections increase cancer risk because viral genetic material or long-term inflammation can disrupt control of cell division.

Separate Cause From Correlation

1. If two diseases occur together, this does not by itself prove that one caused the other.
2. Evidence for causation must show a plausible biological mechanism and rule out other factors that could explain the association.

5.1.3 Pathogens as disease-causing organisms

Pathogens Cause Communicable Disease

1. A pathogen is a microorganism or virus that causes disease.
2. Pathogens cause disease by damaging cells and tissues or by producing toxins.
3. A pathogen can be present before symptoms appear, so an infected person may transmit disease during an incubation period.

Bacteria Are Living Prokaryotic Cells

1. A bacterium is a single prokaryotic cell with cytoplasm, ribosomes, a cell membrane and a cell wall but no nucleus.
2. Its main DNA is a circular loop in the cytoplasm, and some bacteria also contain plasmids.
3. Pathogenic bacteria may reproduce rapidly and release toxins that damage cells.
4. Bacteria reproduce by binary fission, so favourable conditions can produce a rapid increase in bacterial number.
5. Bacterial toxins may damage cell membranes, disrupt enzymes or stimulate water loss from tissues, producing disease symptoms.

Viruses Reproduce Inside Host Cells

1. A virus is a non-cellular particle containing genetic material inside a protein coat.
2. A virus enters a host cell and uses the cell's machinery to make new virus particles.
3. The host cell is damaged or destroyed as the new viruses are released.
4. Virus attachment is specific because surface proteins bind only to complementary receptors on suitable host cells.

Fungi and Protists Are Eukaryotic

1. A pathogenic fungus may grow as thread-like hyphae through tissues and produce spores that spread to new hosts.
2. A pathogenic protist is a single-celled eukaryote that damages host cells, and some are transferred by vectors.
3. Fungal hyphae penetrate tissues, absorb nutrients and damage cells, while spores allow the infection to reach new hosts.
4. Protists are eukaryotic cells with a nucleus and organelles, but pathogenic species live in or on a host and take resources from it.

Compare Pathogens by Cell Structure

1. Bacteria are prokaryotic cells, fungi and protists are eukaryotic, and viruses are not cells.
2. Viruses are much smaller than bacteria and cannot reproduce without a living host cell.

5.1.4 Common infections

Common Infections Damage Specific Tissues

1. The symptoms of an infection depend on the pathogen, the tissue it infects and the damage it causes.
2. You should link each named disease to its pathogen group and its characteristic effect.

Bacterial Diseases

1. Cholera is caused by bacteria that release a toxin in the small intestine, causing severe watery diarrhoea and dangerous dehydration.
2. Tuberculosis is caused by bacteria that mainly infect the lungs, where inflammation and tissue damage reduce gas exchange.
3. Stomach ulcers can be caused by Helicobacter bacteria that damage the stomach lining, allowing acid to injure the tissue beneath.
4. Cholera toxin causes ions to move into the intestinal lumen, and water follows by osmosis, producing severe watery diarrhoea and dehydration.
5. Tuberculosis bacteria can remain in the lungs, where repeated immune responses damage alveoli and reduce the surface available for gas exchange.
6. Helicobacter damages the protective stomach lining, so hydrochloric acid and digestive enzymes reach underlying tissue and form an ulcer.

Fungal and Protist Diseases

1. Chalara ash dieback is a fungal disease of ash trees that causes leaf loss, bark lesions and dieback of shoots and branches.
2. Malaria is caused by a protist carried by female Anopheles mosquitoes, and it damages liver cells and red blood cells.
3. Repeated destruction of red blood cells causes cycles of fever and can lead to anaemia.
4. Ash dieback spreads through fungal spores, causing dark bark lesions, leaf loss and death of shoots as conducting tissues are damaged.
5. The malarial protist first reproduces in liver cells and later in red blood cells, whose repeated rupture produces cycles of fever and anaemia.

Viral Diseases

1. HIV infects and destroys white blood cells, so the immune system becomes less able to control other pathogens.
2. Untreated HIV infection can progress to AIDS, when opportunistic infections and some cancers can become life-threatening.
3. Ebola is a viral disease that causes severe fever and can damage blood vessels, leading to internal and external bleeding.
4. HIV lowers the number of functioning white blood cells over years, while AIDS describes the severe immune-deficiency stage that may follow.
5. Ebola damages several organs and blood vessels, so severe fluid loss and haemorrhage can lead to shock.

Distinguish Disease From Symptom

1. Diarrhoea, fever and bleeding are symptoms, while cholera, malaria and Ebola are diseases.
2. A symptom alone does not identify the pathogen because different diseases can produce similar symptoms.

5.1.5 Spread and prevention of pathogens

Transmission Routes Determine Prevention

1. Transmission is the passing of a pathogen from an infected organism to another organism.
2. Prevention works by breaking the route used by the pathogen.

Water and Airborne Transmission

1. Cholera bacteria spread in water contaminated with faeces from an infected person.
2. Clean drinking water, sewage treatment, handwashing and safe food preparation reduce cholera transmission.
3. Tuberculosis bacteria spread in droplets released when an infected person coughs or sneezes.
4. Good ventilation, covering coughs, testing, isolation and completing antibiotic treatment reduce tuberculosis spread.
5. Ash dieback fungal spores travel through the air and on infected plants.
6. Removing infected material and restricting movement of ash plants reduce the number of spores reaching healthy trees.
7. Sewage treatment separates human waste from drinking water, breaking the faecal-oral route used by cholera bacteria.
8. Ventilation dilutes airborne droplets, while isolation reduces the number of susceptible people exposed to tuberculosis.
9. Removing infected ash trees reduces spore production, and restrictions on plant movement prevent infected material reaching new areas.

Vectors and Direct Contact

1. Malaria spreads when a mosquito vector transfers the protist between human bloodstreams.
2. Insecticide-treated bed nets, repellents, indoor insecticides and removing standing water reduce mosquito bites or breeding.
3. Helicobacter bacteria can spread by oral transmission through saliva or food and water contaminated with faeces.
4. Handwashing, safe water and hygienic food preparation reduce oral transmission.
5. Mosquito control works at different stages: removing standing water reduces breeding, insecticide kills vectors and bed nets prevent bites.

Body Fluids Spread Ebola

1. Ebola virus spreads through direct contact with infected blood or other body fluids and contaminated objects.
2. Isolation, protective clothing, disinfection, safe handling of bodies and contact tracing reduce exposure to infected fluids.
3. Contact tracing identifies people exposed to Ebola before they unknowingly pass infected fluids to others.

Population Controls Add Protection

1. Vaccination reduces the number of susceptible hosts when a suitable vaccine is available.
2. Rapid diagnosis and treatment can shorten the time an infected person passes on a pathogen.

Evaluate a Control Method

1. A control may be effective but limited by cost, access, public compliance or the pathogen's ability to spread before symptoms appear.
2. Using several controls together usually blocks more than one stage of transmission.
3. A prevention programme is stronger when it combines controls because no single measure is completely effective.

5.1.6 Lifecycle of a virus

Viruses Use Host Cells to Reproduce

1. A virus cannot reproduce independently because it lacks the cellular machinery needed to copy genetic material and make proteins.
2. It attaches to a matching receptor on a host cell and transfers its genetic material into the cell.

The Lytic Pathway Destroys the Cell

1. The virus attaches to a suitable host cell and inserts or releases its genetic material.
2. Viral genes take control of the cell's enzymes and ribosomes.
3. The host cell copies viral genetic material and makes viral proteins.
4. New virus particles assemble inside the cell.
5. The host cell lyses, releasing many new viruses that can infect other cells.
6. During attachment, viral surface proteins bind to complementary receptors on a suitable host cell, which determines host-cell specificity.
7. After entry, viral genetic material redirects host enzymes and ribosomes to copy the genome and synthesise capsid proteins.
8. During assembly, copied genetic material is packaged into new protein coats before the particles are released.
9. Lysis kills the host cell as new virus particles escape, so repeated cycles damage tissue and produce symptoms.

The Lysogenic Pathway Delays Lysis

1. Viral genetic material enters the host cell and becomes incorporated into the host DNA.
2. The viral DNA is copied whenever the host cell copies its own DNA and divides.
3. The host cell may continue functioning without producing complete virus particles.
4. A trigger can activate the viral genes, causing the infection to switch to the lytic pathway.
5. In the lysogenic pathway, integrated viral DNA is copied with host DNA without immediate production of complete virus particles.
6. Environmental stress or another trigger can activate integrated viral genes and switch the infection into the lytic pathway.

Compare the Two Pathways

1. Both pathways begin with viral genetic material entering a host cell.
2. The lytic pathway immediately makes new viruses and destroys the host cell.
3. The lysogenic pathway inserts viral material into host DNA and may remain without cell lysis for many divisions.

Explain Symptoms From Cell Damage

1. Lysis damages tissues because many infected cells are destroyed.
2. The immune response to infected cells can also cause inflammation and other symptoms.

5.1.7 Spread and prevention of STIs

STIs Spread Through Sexual Contact or Body Fluids

1. A sexually transmitted infection is an infection passed mainly through sexual contact or exchange of infected body fluids.
2. An infected person may have no symptoms and can still transmit the pathogen.

Chlamydia Is a Bacterial STI

1. Chlamydia is caused by bacteria and is transmitted during unprotected sexual contact.
2. It often causes no symptoms, but it may cause pain when urinating or unusual discharge.
3. Untreated infection can spread through the reproductive system and cause pelvic inflammatory disease and infertility.
4. Barrier contraception reduces transmission, while testing and antibiotics identify and treat infected people and their partners.
5. Chlamydia infects cells lining the reproductive tract and may spread to the uterus and oviducts, where inflammation and scarring can block an oviduct.

HIV Damages Immune Defence

1. HIV is present in blood, semen, vaginal fluids and breast milk.
2. It can spread through unprotected sex, shared needles, infected blood, and from mother to baby during pregnancy, birth or breastfeeding.
3. HIV infects and destroys white blood cells, weakening the immune system.
4. AIDS is the late stage of untreated HIV infection, when severe immune damage allows opportunistic infections and some cancers to develop.
5. HIV transmission requires infected fluid to reach another person's bloodstream or vulnerable tissue.

Several Measures Reduce HIV Transmission

1. Condoms reduce exchange of infected sexual fluids.
2. Sterile needles prevent transfer of infected blood.
3. Screening donated blood prevents infected blood products being used.
4. Testing allows infected people to begin treatment and take steps to avoid transmission.
5. Antiretroviral drugs reduce viral replication, protecting white blood cells and lowering the amount of virus in the blood.
6. Treatment during pregnancy and birth greatly reduces transmission from mother to baby.
7. Antiretroviral therapy uses combinations of drugs to inhibit stages of viral replication, reducing viral load and protecting white blood cells.
8. A lower viral load reduces transmission risk, but treatment does not remove the virus from the body.
9. Condoms reduce exchange of sexual fluids, sterile needles prevent blood-to-blood transfer and screened donations prevent infected transfusions.

Prevention Does Not Depend on Symptoms

1. Regular testing is necessary because chlamydia and early HIV infection may be asymptomatic.
2. Partner notification allows exposed people to be tested and treated.
3. A person with asymptomatic chlamydia can transmit bacteria without knowing, which is why screening and partner notification are needed.