Viral, bacterial, fungal and protist diseases

What you'll learn

This revision guide covers the four main types of pathogens that cause infectious diseases in humans, animals and plants. You will learn to identify specific diseases caused by viruses, bacteria, fungi and protists, understand how they spread, their symptoms, and methods of prevention and treatment. This topic is essential for Paper 1 and forms part of the Infection and Response unit.

Key terms and definitions

Pathogen — A microorganism that causes disease (includes bacteria, viruses, fungi and protists)

Communicable disease — An infectious disease that can be spread between individuals, caused by pathogens

Vector — An organism that spreads disease without getting the disease itself (e.g. mosquitoes spreading malaria)

Antibiotic — A drug that kills bacteria or prevents their growth; ineffective against viruses

Antiviral — A drug that slows down or stops viral reproduction; cannot destroy viruses completely

Vaccine — A preparation containing dead or weakened pathogens that stimulates the immune system to produce antibodies

Immunisation — The process of making a person immune to a disease, usually by vaccination

HIV (Human Immunodeficiency Virus) — A virus that attacks white blood cells, eventually leading to AIDS if untreated

Core concepts

Viral diseases

Viruses are the smallest pathogens and can only reproduce inside living host cells. They cause disease by invading cells, using cellular machinery to replicate, then bursting out and destroying the host cell.

Measles

Measles is a highly contagious viral disease spread through droplet infection when an infected person coughs or sneezes. The virus is carried in tiny droplets of moisture in the air.

Symptoms:

• High fever lasting several days
• Red skin rash across the whole body
• Cold-like symptoms (runny nose, cough)
• Red, watery eyes

Complications: Measles can be fatal if complications develop, including pneumonia and encephalitis (brain inflammation). In the UK, measles killed approximately 1 in 5000 infected children before vaccination programmes.

Prevention and treatment:

• Vaccination with the MMR vaccine (measles, mumps, rubella) given at 12-13 months and 3-4 years
• No cure exists; treatment focuses on managing symptoms
• Most children are vaccinated, but outbreaks still occur in unvaccinated communities

HIV and AIDS

HIV (Human Immunodeficiency Virus) is transmitted through direct exchange of bodily fluids, particularly blood and sexual fluids.

Methods of transmission:

• Unprotected sexual intercourse
• Sharing needles contaminated with infected blood
• From mother to baby during pregnancy, birth or breastfeeding
• Blood transfusions (very rare in the UK due to screening)

How HIV affects the body:

HIV attacks and destroys white blood cells called T-helper lymphocytes, which coordinate the immune response. As these cells are destroyed over several years, the immune system becomes progressively weaker. Eventually, the body cannot fight off infections that would normally be harmless.

AIDS (Acquired Immune Deficiency Syndrome) is the final stage of HIV infection when the immune system is so damaged that the body becomes vulnerable to opportunistic infections and certain cancers.

Prevention and treatment:

• Use of condoms during sexual intercourse
• Screening of blood donations
• Not sharing needles (needle exchange programmes)
• Antiretroviral drugs cannot cure HIV but suppress viral replication, preventing progression to AIDS
• Pre-exposure prophylaxis (PrEP) for high-risk individuals
• Antiretroviral therapy allows people with HIV to live normal lifespans and reduces transmission to nearly zero

Bacterial diseases

Bacteria are single-celled prokaryotic organisms that can reproduce rapidly inside the body. They cause disease by producing toxins (poisons) that damage cells and tissues, or by directly damaging cells.

Salmonella food poisoning

Salmonella bacteria cause bacterial food poisoning when contaminated food is consumed.

Sources of infection:

• Undercooked chicken, eggs and meat
• Food prepared in unhygienic conditions
• Food contaminated by people carrying the bacteria

Symptoms:

• Fever
• Abdominal cramps and pain
• Vomiting
• Diarrhoea
• Symptoms typically appear 6-72 hours after infection

Prevention:

• Cooking food thoroughly (especially poultry)
• Washing hands before handling food
• Keeping raw meat separate from ready-to-eat foods
• In the UK, poultry are vaccinated against Salmonella to prevent spread
• Proper refrigeration of food

Treatment:

• Most cases resolve without treatment within a week
• Fluid replacement to prevent dehydration
• Antibiotics only used in severe cases

Gonorrhoea

Gonorrhoea is a sexually transmitted disease (STD) caused by bacteria.

Transmission:

• Unprotected sexual intercourse (vaginal, anal or oral)
• Direct genital contact

Symptoms:

• Thick yellow or green discharge from genitals
• Pain when urinating
• Many infected people (especially women) show no symptoms initially
• Untreated gonorrhoea can cause pelvic inflammatory disease, infertility and ectopic pregnancies

Prevention and treatment:

• Barrier contraception (condoms)
• Screening of sexually active individuals
• Treated with antibiotics, though antibiotic resistance is an increasing problem
• Some strains are now resistant to multiple antibiotics
• Contact tracing to identify and treat partners

Fungal diseases

Fungi can cause diseases in both plants and animals. Fungal diseases spread through spores, which can be carried by wind, water or direct contact.

Rose black spot

Rose black spot is a fungal disease affecting roses and other plants in the Rosaceae family.

Transmission:

• Fungal spores spread by wind and water splashes from rain or watering
• Can survive on fallen leaves over winter
• Thrives in warm, wet conditions

Symptoms and effects:

• Purple or black spots develop on leaves
• Leaves turn yellow around the spots
• Premature leaf drop occurs
• Reduced photosynthesis due to loss of leaf area and chlorophyll damage
• Weakened plant with reduced growth
• Fewer flowers produced

Treatment and prevention:

• Remove and destroy affected leaves (burn or dispose of; do not compost)
• Spray with fungicides containing chemicals like tebuconazole
• Plant resistant rose varieties
• Ensure good air circulation around plants
• Avoid overhead watering which spreads spores

Protist diseases

Protists are single-celled eukaryotic organisms. Many are parasites that live in or on host organisms, causing harm.

Malaria

Malaria is one of the most serious global health problems, caused by Plasmodium protists and spread by mosquitoes as vectors.

Life cycle and transmission:

1. Female Anopheles mosquitoes bite an infected person and take up Plasmodium protists with the blood
2. The protists reproduce sexually inside the mosquito
3. When the mosquito bites another person, Plasmodium protists are injected with saliva (which contains anticoagulants)
4. The protists travel to the liver where they reproduce asexually
5. They enter red blood cells, multiply, then burst out causing cell damage
6. This cycle repeats every 48-72 hours, causing recurring fever episodes

Symptoms:

• Recurring episodes of fever (every 2-3 days when red blood cells burst)
• Shaking and chills
• Headaches and muscle pain
• Fatigue and weakness
• Can be fatal, especially in children and pregnant women

Global distribution:

• Common in tropical and sub-tropical regions (Africa, South Asia, Central and South America)
• Approximately 200 million cases annually
• Over 400,000 deaths per year, mostly children under 5 in Africa
• Climate change may extend the range of Anopheles mosquitoes

Prevention:

• Mosquito nets (especially insecticide-treated bed nets) to prevent bites during night when Anopheles mosquitoes feed
• Insect repellent containing DEET
• Long-sleeved clothing at dusk and dawn
• Antimalarial drugs for travellers to endemic areas
• Draining standing water where mosquitoes breed
• Indoor insecticide spraying
• Research into vaccines (RTS,S vaccine now being piloted in African countries)

Treatment:

• Antimalarial drugs such as artemisinin-based combination therapies
• Drug resistance is an increasing problem in some Plasmodium species
• Early diagnosis and prompt treatment essential to prevent complications

Worked examples

Example 1: Explaining disease transmission (4 marks)

Question: Explain how malaria is transmitted from person to person.

Mark scheme answer:

• Malaria is caused by Plasmodium protists (1 mark)
• These are transferred by mosquitoes acting as vectors (1 mark)
• Female Anopheles mosquitoes bite an infected person and take up the protist (1 mark)
• When the mosquito bites another person, the protists are injected into their blood / protists reproduce inside the mosquito (1 mark)

Examiner tip: Notice that "transferred by mosquitoes" alone would not gain full marks; you must state that mosquitoes are vectors and explain the process of transmission through biting.

Example 2: Comparing pathogens (6 marks)

Question: Compare bacterial and viral diseases. Refer to named diseases in your answer.

Model answer:

• Viruses such as measles and HIV are much smaller than bacteria such as Salmonella (1 mark)
• Viruses can only reproduce inside living host cells whereas bacteria can reproduce outside cells / in the body or on surfaces (1 mark)
• Bacterial diseases can be treated with antibiotics but viral diseases cannot (1 mark)
• For example, gonorrhoea is treated with antibiotics but HIV requires antiretroviral drugs (1 mark)
• Both can spread between people but use different mechanisms, for example HIV through body fluids and Salmonella through contaminated food (1 mark)
• Both types can be prevented by vaccination, for example measles vaccine and the development of HIV vaccines (1 mark)

Examiner tip: "Compare" questions require you to make comparative statements (using words like "whereas", "but", "while") or to give features of both items being compared. Simply describing each separately will limit your marks.

Example 3: Explaining prevention methods (5 marks)

Question: A student says: "We should stop vaccinating chickens against Salmonella because we can just cook chicken properly instead." Evaluate this statement.

Model answer:

Arguments against the student's view:

• Vaccination of chickens prevents Salmonella bacteria from spreading through flocks, reducing contamination of eggs and meat (1 mark)
• Humans can still be infected through cross-contamination even if chicken is cooked, for example through contact with raw meat or contaminated surfaces (1 mark)
• Some people are more vulnerable to Salmonella (elderly, children, immunocompromised) so prevention is better than relying on cooking (1 mark)

Arguments supporting the student's view:

• Thorough cooking does kill Salmonella bacteria effectively (1 mark)
• Vaccination programmes have costs for farmers (1 mark)

Examiner tip: "Evaluate" means you must present evidence for both sides and may reach a conclusion about which is more important or correct, though a conclusion is not always required for full marks.

Common mistakes and how to avoid them

• Saying antibiotics can treat viral diseases: Remember that antibiotics only work on bacteria. Viral diseases require antivirals (which slow reproduction) or management of symptoms. HIV is treated with antiretroviral drugs, not antibiotics.

• Confusing HIV and AIDS: HIV is the virus that causes the infection. AIDS is the syndrome (collection of symptoms) that develops when HIV has severely damaged the immune system. A person can have HIV for many years without developing AIDS if they receive treatment.

• Stating mosquitoes cause malaria: Mosquitoes are vectors that spread malaria; they don't cause it. The disease is caused by Plasmodium protists. Vectors carry the pathogen but don't suffer from the disease themselves.

• Forgetting to explain how reduced photosynthesis affects the plant: When discussing rose black spot, don't just state that photosynthesis is reduced. Explain that this means less glucose is produced, so less energy is available for growth, leading to stunted plants and fewer flowers.

• Not providing enough detail about disease transmission: Simply stating "through the air" or "through blood" is often insufficient. Use specific terminology like "droplet infection", "direct exchange of bodily fluids", or "contaminated food".

• Mixing up different types of pathogens: Be precise about which diseases are caused by which pathogen type. Don't say measles is bacterial or Salmonella is viral – learn which specific pathogen causes each disease on the specification.

Exam technique for "Viral, bacterial, fungal and protist diseases"

• For "describe" questions about symptoms or transmission: Use bullet points or numbered steps in your answer. Award marks typically come one per distinct point, so "severe headache and high fever" might only get 1 mark if both are symptoms of fever.

• When asked to "explain" prevention methods: Don't just name the method; state how it works to prevent infection. For example, not just "use condoms" but "condoms provide a barrier that prevents exchange of bodily fluids containing the pathogen".

• Extended response questions (4-6 marks): Structure your answer logically. For disease transmission, describe the pathway from infected person → vector or transmission route → uninfected person. For impacts, explain immediate effects and consequences (e.g., leaf spots → reduced chlorophyll → reduced photosynthesis → less growth).

• Link cause and effect clearly: Use words like "therefore", "which causes", "resulting in", "leading to" to show you understand the chain of biological events, not just memorised facts.

Quick revision summary

Four types of pathogens cause communicable diseases: viruses (measles, HIV), bacteria (Salmonella, gonorrhoea), fungi (rose black spot) and protists (malaria). Each spreads differently—through droplets, bodily fluids, contaminated food, vectors or spores. Bacterial diseases are treated with antibiotics; viral diseases require antivirals or symptom management. Prevention includes vaccination, hygiene measures, barrier contraception, destroying infected material, and controlling vectors like mosquitoes. Understand specific transmission routes, symptoms and prevention for each named disease on the AQA specification.