Pathogens: Viral, Bacterial, Fungal & Protist Diseases
Communicable diseases are infectious illnesses caused by microorganisms called pathogens, which include viruses, bacteria, fungi, and protists that can spread between organisms. Bacteria reproduce rapidly by binary fission and release toxins that damage tissues, whereas viruses invade host cells and replicate inside them until the cell bursts (lysis). GCSE Biology specifications require students to know the exact pathogen type, symptoms, transmission method, and prevention for seven specific human and plant diseases.
- Viral Diseases: Measles & HIV
- Measles: spread by droplet inhalation from coughs/sneezes; causes fever and red skin rash; can be fatal; prevented by MMR vaccination. HIV: spread by sexual contact or exchanging bodily fluids (e.g., sharing needles); causes flu-like illness initially then attacks immune cells (leading to late-stage HIV / AIDS); controlled with antiretroviral drugs.
- Viral Plant Disease: Tobacco Mosaic Virus (TMV)
- A widespread plant pathogen affecting tobacco and tomato plants. Spread by direct contact between diseased plants or via gardeners' hands/tools. Causes a mosaic pattern of leaf discolouration, reducing chlorophyll and stunting growth due to reduced photosynthesis.
- Bacterial Diseases: Salmonella & Gonorrhoea
- Salmonella: food poisoning spread by eating undercooked poultry or food prepared in unhygienic conditions; toxins cause fever, abdominal cramps, vomiting, and diarrhoea; controlled in the UK by vaccinating poultry. Gonorrhoea: sexually transmitted disease (STD); causes thick yellow/green discharge and pain urinating; treated with antibiotics (though antibiotic-resistant strains have evolved) and prevented by barrier contraception (condoms).
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Mitochondria
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Site of aerobic respiration, releasing energy (ATP) for the cell. Folded inner membrane increases surface area for enzymes.
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Magnification = Image Size ÷ Actual Size (I = A × M). Convert cm → mm (×10) → µm (×1000) before dividing!Key Concepts & Exam Definitions (6)
- Viral Diseases: Measles & HIV
- Measles: spread by droplet inhalation from coughs/sneezes; causes fever and red skin rash; can be fatal; prevented by MMR vaccination. HIV: spread by sexual contact or exchanging bodily fluids (e.g., sharing needles); causes flu-like illness initially then attacks immune cells (leading to late-stage HIV / AIDS); controlled with antiretroviral drugs.
- Viral Plant Disease: Tobacco Mosaic Virus (TMV)
- A widespread plant pathogen affecting tobacco and tomato plants. Spread by direct contact between diseased plants or via gardeners' hands/tools. Causes a mosaic pattern of leaf discolouration, reducing chlorophyll and stunting growth due to reduced photosynthesis.
- Bacterial Diseases: Salmonella & Gonorrhoea
- Salmonella: food poisoning spread by eating undercooked poultry or food prepared in unhygienic conditions; toxins cause fever, abdominal cramps, vomiting, and diarrhoea; controlled in the UK by vaccinating poultry. Gonorrhoea: sexually transmitted disease (STD); causes thick yellow/green discharge and pain urinating; treated with antibiotics (though antibiotic-resistant strains have evolved) and prevented by barrier contraception (condoms).
- Fungal Plant Disease: Rose Black Spot
- A fungal disease where purple or black spots develop on rose leaves, which then turn yellow and drop early. This reduces the leaf surface area and chlorophyll available for photosynthesis, stunting growth. Spread by water droplets (rain splash) or wind; treated using fungicides and by removing and burning infected leaves.
- Protist Disease: Malaria
- Caused by a eukaryotic protist pathogen whose life cycle includes the female Anopheles mosquito vector. Causes recurrent, severe episodes of fever and can be fatal. Prevented by stopping mosquito vectors breeding (draining stagnant water) and using mosquito nets and insect repellent.
- Non-specific human defence systems
- Physical and chemical barriers that prevent pathogens entering the body: 1. Skin (physical barrier that forms scabs via platelet clotting when cut and secretes antimicrobial sebum). 2. Nose hairs & mucus. 3. Trachea & bronchi (goblet cells produce mucus; cilia sweep it to the throat). 4. Stomach (hydrochloric acid at pH 2 kills ingested pathogens).
Quick-Fire Flashcards (6)
Click any card to check the answer, then mark it as Known to track your progress.
Name the three viral diseases required for GCSE Biology.
Name the two bacterial diseases required for GCSE Biology.
What type of pathogen causes Rose Black Spot and how is it treated?
What is a disease vector? Give an example.
Why does HIV eventually lead to AIDS if left untreated?
How is the spread of Salmonella controlled in the UK?
Exam-Style Questions & Mark Schemes (4)
Write or think through your answer first, then click Check Mark Scheme to see the exact points examiners award marks for.
- Q1
Compare how bacteria and viruses reproduce inside the human body and explain how each causes symptoms of disease. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Bacteria reproduce rapidly outside cells by binary fission (asexual cell division) [1 mark]. 2. Bacteria make us feel ill by producing and releasing poisonous toxins that damage tissues [1 mark]. 3. Viruses are much smaller and cannot reproduce on their own; they invade host cells and use the cell's biochemistry to replicate copies of themselves [1 mark]. 4. The host cell eventually bursts open (lysis) to release the new viruses, and this cell damage is what makes us feel ill [1 mark].
- Q2
A gardener notices purple-black spots on the leaves of their rose bushes and stunted flower growth. Identify the disease, explain why growth is stunted, and describe two ways to prevent it spreading to healthy bushes. [5 marks]
Click “Check Mark Scheme” to compare your answer, 1. The disease is Rose Black Spot (caused by a fungus) [1 mark]. 2. The spots and early leaf drop reduce the surface area and amount of chlorophyll available to absorb light energy [1 mark]. 3. This lowers the rate of photosynthesis, so the plant produces less glucose needed to synthesise proteins and cellulose for growth [1 mark]. 4. Prevention 1: Treat the plants with chemical fungicides [1 mark]. 5. Prevention 2: Remove and burn (destroy) the infected leaves so fungal spores cannot be spread by wind or rain splash [1 mark].
- Q3
Explain why a doctor will prescribe antibiotics for a patient with gonorrhoea, but will prescribe antiretroviral drugs instead for a patient with HIV. [4 marks]
Click “Check Mark Scheme” to compare your answer, 1. Gonorrhoea is caused by a bacterium, which has a cell wall and its own metabolic machinery that antibiotics can target and destroy [1 mark]. 2. However, some strains of gonorrhoea have evolved resistance to penicillin, so specific effective antibiotics must be chosen [1 mark]. 3. HIV is caused by a virus, which replicates inside the patient's own immune cells (white blood cells) [1 mark]. 4. Antibiotics cannot kill viruses inside host cells, so antiretroviral drugs are used to stop the virus replicating and prevent the immune system from being destroyed [1 mark].
- Q4
A single bacterium divides by binary fission once every 20 minutes under optimum conditions. Calculate how many bacteria will be present after 4 hours. Give your answer in standard form. [3 marks]
Click “Check Mark Scheme” to compare your answer, 1. Number of divisions in 4 hours (240 minutes) = 240 / 20 = 12 divisions [1 mark]. 2. Number of bacteria = 1 x 2¹² = 4,096 bacteria [1 mark]. 3. In standard form = 4.096 x 10³ (or 4.1 x 10³) bacteria [1 mark].
Common Exam Questions (4)
Why can antibiotics cure bacterial infections like Salmonella or gonorrhoea, but not viral infections like measles or HIV?
Antibiotics work by disrupting bacterial cell structures or metabolic processes (such as breaking down bacterial cell walls). Viruses do not have cell walls or their own metabolic machinery; furthermore, viruses live and replicate inside the host's own cells, so antibiotics cannot reach or destroy them without damaging human cells.
How do bacteria and viruses make us feel ill?
Bacteria divide rapidly inside the body by binary fission and secrete poisonous chemicals called toxins that damage tissues and cause symptoms like fever and cramps. Viruses invade host cells and hijack the cell's machinery to make thousands of viral copies, eventually causing the host cell to burst open and die.
How is malaria transmitted and how can its spread be prevented?
Malaria is caused by a single-celled protist parasite. The female Anopheles mosquito acts as a vector, picking up the protist when feeding on an infected person's blood and injecting it into the next person it bites. Spread is prevented by using insecticide-treated mosquito nets, draining stagnant water where mosquitoes breed, and taking antimalarial drugs.
How does Tobacco Mosaic Virus (TMV) reduce plant growth?
TMV causes a distinctive 'mosaic' pattern of yellow discolouration on plant leaves by destroying chloroplasts and chlorophyll. With less chlorophyll, the plant absorbs less light energy, which reduces the rate of photosynthesis and limits the glucose available to make proteins and cellulose for growth.
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Related study guides
The Immune System & Vaccination
The human body defends itself against disease-causing microorganisms (pathogens) using non-specific physical and chemical barriers alongside the specific immune system. White blood cells destroy invading pathogens through phagocytosis, antibody production, and antitoxin release, while vaccination establishes long-term immunological memory.
Monoclonal Antibodies & Plant Disease Detection
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Cancer, Lifestyle Risk Factors & Non-Communicable Disease
Non-communicable diseases cannot be spread between organisms because they are not caused by pathogens; instead, they arise from interactions between genetic predisposition, environmental exposures, and lifestyle risk factors. Cancer occurs when mutations in a cell's DNA disrupt the normal regulation of the cell cycle and mitosis, leading to uncontrolled cell division and the formation of benign or malignant tumours. Examiners frequently test the distinction between benign and malignant tumours, the difference between correlation and causal mechanisms, and epidemiological data analysis.