4.3 Infection and response

4.3.1 Communicable diseases

4.3.1.1 Communicable (infectious) diseases

Communicable Disease

• Communicable diseases spread from one organism to another.
• They are contagious or infectious diseases.

Pathogens

• Communicable diseases spread via pathogens
• Pathogens are microorganisms that cause infectious diseases
  • e.g. Human Immunodeficiency Virus causes AIDS

Types of Pathogen

1. Bacteria
2. Viruses
3. Fungi
4. Protists

Bacteria

• Bacterial cells are prokaryotic cells
• They do not have a nucleus, and are 1% the size of animal cells
• Animal, plant and bacterial cells all have a cytoplasm and a cell membrane
• Bacteria divide quickly and spread disease using toxins

Viruses

• Viruses are not cells, but are biological structures made of genetic material surrounded by proteins and lipids
• They invade host cells, reproduce and spread
• Viruses have no cell organelles, but they carry DNA and RNA for replication

Viral Infection

Fungi

• Fungal cells are eukaryotes, but are neither plants or animals
• Fungi can be unicellular or multicellular
• They get their nutrition by breaking down decaying matter
• Fungi have a chitin cell wall (unlike cellulose cell walls in plants)
• Fungi feed saprotrophically
  • They secrete enzymes to digest food, then absorbing the digested molecules
• Fungal cells can join to make hyphae, which can cause infections

Protists

• Everything which doesn’t fit into animals, plants and fungi will be classified as protists
• Many protists are parasites, which often use vectors to transport from organism to organism
  • E.g. the protist Plasmodium Protozoa causes malaria, and uses mosquitos as a vector

Spread of Pathogens

1. Water
2. Direct Contact
3. Air Droplets

Water

• Water contaminated by pathogens can cause infections via bathing or ingesting
  • E.g. Cholera is spread like this, as the water is contaminated by the faeces of the patients

Direct Contact

• Direct contact with sufferers can spread disease
• The type of direct contact will determine whether the pathogen will spread
  • E.g. chicken pox can spread just by touching, while HIV requires sexual contact for spreading

Air Droplets

• Droplet infections are spread in the air through coughs and sneezes
  • E.g. influenza, the common flu, is spread from person to person via droplets

Spread of Pathogens

• Improving Hygiene
• Avoiding Infected Individuals
• Drugs and Medicines
• Vector Control
• Medicines

Improving Hygiene

• Washing hands after using the lavatory and sneezing
• Sneezing into the tissue, then disposing of the tissue
• Cleanliness in food preparation
• Reducing cross-contamination of produce and meats

Avoiding Infected Individuals

• We want to reduce exposure of people to an infected person
  • For example, if a child has chicken pox, the school should advise against them coming in

Drugs and Medicines

• Some drugs will help to kill pathogens quickly once they have entered the body
• They reduce the spread of infection
  • E.g. antibiotics work against bacterial pathogens

Vector Control

• Many communicable diseases spread via vectors, which are carriers
• We can eradicate or control the population of vectors
  • E.g. using mosquito spray when going to malaria- risk areas

Vaccination

• People can be vaccinated against many diseases
  • E.g. the Hepatitis B vaccination will help to protect against Hepatitis B infection
• The vaccination doesn’t stop the infection from happening, but it helps the body respond quickly

4.3.1.2 Viral diseases

Measles

• Measles is a viral infection caused by the measles virus
• It is spread through the air in droplets (coughs and sneezes)
• Those suffering from measles can get a red rash, a high fever and small white spots inside the mouth

Measles: Symptoms

Measles: Complications

Measles: Vaccinations

Measles

• Complications of measles can lead to brain infections such as encephalitis (inflammation of the brain)
• Most children are vaccinated against measles in the MMR vaccine
• Deaths from measles have been falling due to increased vaccinations

HIV: Mechanism

• The HIV virus can launch an attack on the body’s immune system and weaken it to pathogens
• In HIV sufferers the body’s ability to produce antibodies is slower

Human Immunodeficiency Virus (HIV)

• HIV can progress to Acquired Immune Deficiency Syndrome (AIDS), where the body’s immunity becomes very weak
• There is currently no vaccine or cure – it is controlled by antiretroviral drugs
• Extra precautions should be taken to avoid infections from other diseases

HIV: Spread

• HIV spreads via body fluids (blood, semen)
• It can therefore spread via needle sharing in drug users, unprotected sexual intercourse, and wound bleeding.

HIV: Pathophysiology

• Early symptoms of HIV are similar to the flu
• HIV can progress to Acquired Immune Deficiency Syndrome (AIDS), where the body’s immunity becomes very weak

HIV: Treatment

• There is currently no vaccine or cure for HIV
• However, we can control HIV using antiretroviral drugs
• Extra precautions should be taken to avoid infections from other diseases, because the immune system is weak

HIV: Prevention

Tobacco Mosaic Virus (TMV)

TMV is a plant pathogen that affects tobacco and other tobacco like plants

Tobacco Mosaic Virus: Mechanism

Tobacco Mosaic Virus: Mechanism

Tobacco Mosaic Virus: Spread

Tobacco Mosaic Virus (TMV)

• It infects chloroplasts and turns the plants yellow, affecting photosynthesis
• TMV interrupts plant growth and leads to a reduction in crop yield
• TMV can be spread by plants touching one another
• Infected plants need to removed immediately, and hands should be washed between planting

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4.3.1.3 Bacterial diseases

Salmonella: Food Poisoning

• Salmonella is spread through eating contaminated food
• Food can be contaminated by bacteria present on the food, often due to unhygienic food preparation
• This leads to food poisoning

Salmonella: Prevention

• Salmonella is prevented via vaccination programmes

Salmonella: Symptoms

• Fever
• Diarrhoea
• Vomiting
• Stomach cramps

Gonorrhoea: Spread

• Gonorrhoea is an STD transmitted by sexual contact and spread by bacteria
• The spread can be prevented using antibiotics, or by barrier contraception
• Antibiotic resistant strains of the bacteria have developed

Gonorrhoea: Preventing Spread

The spread can be prevented using:

Gonorrhoea: Symptoms

• Pain during urination
• Characterised by the discharge of a yellow / green fluid from the vagina or penis

4.3.1.4 Fungal diseases

Rose Black Spot: Symptoms

• It attacks the leaves, giving them black or purple spots
• The leaves turn yellow and fall off
• This reduces the plants capability for photosynthesis

Rose Black Spot: Spread

• Waterborne transmission
• Airborne transmission
• Direct transmission

Rose Black Spot: Treatment

• Rose black spot is treated by fungicides
• The infected leaves are also removed

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4.3.1.5 Protist diseases

Malaria: Spread

Malaria is caused by the Plasmodium protist

Spread

• Malaria is caused by the Plasmodium protist
• An infected mosquito passes on plasmodium spores to the human blood
• The spores travel in the blood to the liver
• Another mosquito can bite the same individual and pass the spores on to another human – transmissions

Malaria: Symptoms

• Feverish symptoms
• Chills and sweats
• Headaches
• Diarrhoea
• Vomiting

Malaria: Prevention

Controlling the mosquito population and stopping biting can reduce the spread of malaria

Malaria: Prevention

4.3.1.6 Human defence systems

Types of Defence

• Non-Specific: the defence fights against pathogens regardless of what type of pathogen or disease it causes 
• Specific: each antibody is specific to a particular pathogen
  • E.g the antibodies against the HIV virus is different to the antibodies against tuberculosis

Primary Defence

• Primary defence systems are the first line of defence, and are non-specific defences
• They aim to stop entry of pathogens into the body
• These defences aren’t specific against specific pathogens

1. Skin

• Skin forms the primary physical defence, by forming a barrier
  • The skin is made up of dead cells, with keratin strengthening this barrier
• Skin also has a chemical defence
  • There can be secretion of an antimicrobial sebum onto the surface of the skin

2. Nose, Trachea, Bronchi

• The oral passages, nasal passages, eyes and ears are exposed to pathogen invasion
• Goblet cells secrete mucous, which traps pathogens and dust
• Ciliated cells sweep them back out

3. Stomach

• The stomach secretes gastric acid, which kills pathogens in food
• Pepsin in the stomach are adapted to work in the acidic conditions of the stomach

The Immune System

• White blood cells make up the immune system
• WBCs help protect against infection and foreign pathogens
• There are two types: Phagocytes and Lymphocytes

Types of White Blood Cell

• Phagocytes engulf and eat pathogens in phagocytosis.

• Lymphocytes produce antibodies to fight against pathogens

Phagocytosis

Phagocytes are non-specific white blood cells

Phagocytosis

1. A phagocyte detects a pathogen
2. The phagocyte binds to the pathogen
3. The phagocyte engulfs the pathogen
4. Lysosomes break down the pathogen
5. The debris is excreted

Lymphocytes

• Lymphocytes are specific white blood cells

Lymphocytes

1. Lymphocytes bind to antigens
2. Antibodies are produced
3. Antibodies defend against pathogens

4.3.1.7 Vaccination

Vaccination

• A vaccination injects an inactive pathogen into the body
• This activates an immune response, producing memory cells for storage
• Dead pathogens are given with the same antigens as the normal pathogen

How Long Do Vaccines Take To Work?

• Vaccinations allow the body to eliminate a live infection for the first time within seven days
• The peak of the immune response occurs within the first three days

Herd Immunity

• Mass vaccination contributes to herd immunity
• The virus essentially disappears because it is unable to pass from person to person
• We vaccinate against diseases which a person has not encountered before, usually early in life

4.3.1.8 Antibiotics and painkillers

Painkillers and Antibiotics

• Painkillers relieve symptoms, yet do not get rid of infection
  • E.g. Ibuprofen
• Antibiotics reduce bacterial growth and can stop its spread by treating the underlying issue to relieve symptoms
  • E.g. Penicillin

Antibiotics

• Antibiotics specifically target and kill infective bacteria in the body
• Different bacteria have different properties, so must be targeted by certain antibiotics
• Antibiotics have saved many lives over the years
• Everyday treatments, such as surgery, would be impossible without antibiotics

Antibiotic Resistance

Antibiotic Resistance

• Bacteria gain resistance due to natural selection:

1. A mutation causes resistance
2. A selection pressure is applied
3. The resistance alleles are selected for
4. Bacteria evolve to become resistant

Causes of Antibiotic Resistance

• Overuse of antibiotics in medicine and farming
  • Antibiotics add the selection pressure, which speeds up the process on natural selection
• Incompletion of antibiotic course
  • Not all the bacteria are killed, and the ones left over tend to be the resistant ones

Viral Drugs

• Antibiotics can only kill bacteria, not viruses
• Antiviral drugs are required to kill viruses
• These drugs are not as common, as they often damage host cells whilst killing viruses

4.3.1.9 Discovery and development of drugs

Finding New Treatments

• Research is constantly being done to discover new drugs or find better treatments
• This is particularly important as antibiotic resistance grows
• Historically, most drugs have been extracted from plants and microorganisms
• In modern times, most drugs are synthesised by chemists
• Finding treatments for new diseases
• Finding treatments for known diseases with no cure
• Finding substitute treatments to replace current ones
• Finding better treatments to replace current ones

Digitalis

• Digitalis is extracted from foxglove plant leaves
• Digoxin is extracted from the plant
• This is used to stimulate heart muscle and increase heart rate

Aspirin

• Aspirin is derived from Willow tree bark
• Salicylic acid in this tree bark is the main ingredient in aspirin
• Aspirin is used to reduce pain and inflammation

Penicillin

• Penicillin was actually discovered by accident by Alexander Fleming
• A mould grew in his lab due to poor hygiene
• Fleming found that the penicillin could be used to make an antibiotic

Drug Testing: Dosage

• It is important to work out the optimum dosage for a drug
• Too low a dose will mean that the drug is ineffective
• Too high a dose can lead to dangerous side effects and toxicity

Drug Testing: Toxicity

• It is important to check for toxic side effects of the drug
• Some drugs might lead to mutations and increase risk of cancer
• We need to check for both short and long term side effects

Drug Testing: Efficacy

• It is important to test how effective the drug is against the disease
• It needs to have a significant benefit in treating patients before it can be approved

Clinical Trials

• Clinical trials are research studies used to investigate scientific theories
• Many do not get approved for national use
• It can take several years until a drug is approved

Preclinical Testing – Lab Testing

• Preclinical testing involves computer modelling and testing in vivo
• This stage aims to make sure that the drug is safe and effective before introducing it into animals or humans

Preclinical Testing – Animal Testing

• Once the lab results show safety, the drug is tested on animals
• Animal testing poses ethical problems, yet it is still necessary in the UK before the introduction of any drug
• After passing this stage, clinical testing on human healthy volunteers is done

Clinical Testing – Phase 1

• The first stage of clinical testing of a drug on humans uses a very low dose
• The number of patients is quite small (15-20 participants)
• The aim is to check if the treatment is safe and to find the right dose

Clinical Testing – Phase 2

• The second stage of clinical testing involves testing how well the drug works in actually treating patients – the efficacy
• This is a larger test involving more participants (20-150 participants)

Clinical Testing – Phase 3

• The third stage of clinical testing involves comparison of the drug to the current treatment in a large trial
• This trial can involve 100s or 1000s of participants
• Some trials are double blind, and patients are either given a placebo or the medication