Infectious Diseases (Leaving Cert Biology): Revision Notes
Infectious Diseases
What are infectious diseases?
Infectious diseases are illnesses caused by pathogens - harmful microorganisms that invade and multiply in the body. The main types of pathogens include bacteria, viruses, fungi, and parasites. Understanding how these diseases work, spread, and can be treated is crucial for maintaining public health.
A parasite is an organism that takes in food from a live host and usually causes harm. There are two main types:
Types of Parasites:
- Obligate parasite: can only take in food from a live host
- Facultative parasite: can take in food from a live or dead host
Treating microbial diseases
The treatment for a microbial disease depends on the type of microbe causing the disease. Different pathogens require different treatment approaches.
Antibiotics
Antibiotics are used to kill or inhibit the growth of bacteria. They work by targeting specific bacterial processes or structures that are different from human cells. However, antibiotics are only effective against bacterial infections, not viral infections.
Antibiotics are only effective against bacterial infections - they cannot treat viral infections like the common cold or flu.
Antiviral drugs
Antiviral drugs can be used to treat some (but not all) virus infections. For example, acyclovir (sold as Zovirax) is used to treat the virus that causes cold sores. These drugs work by interfering with the virus's ability to replicate inside host cells.
Antifungal drugs
Antifungal drugs can be used to treat fungal infections. These medications target the unique features of fungal cells, such as their cell wall composition, which differs from human cells.
Immunotherapy
Immunotherapy is the use of the body's immune system to fight infections (and cancers). For example, monoclonal antibodies may be used to stimulate the immune response against viruses. Monoclonal antibodies (mAbs) are artificially produced identical copies of a single antibody designed to target specific pathogens.
Bacteriophages
Bacteriophages can be used to treat specific bacterial infections. These are viruses that specifically infect and destroy bacteria, offering a potential alternative to antibiotics.
Virus replication
The structure and functions of viruses make them unique pathogens. The term replication (instead of reproduction) is used for viruses because viruses are not cells. Viruses cannot replicate by themselves - they can only multiply by entering a host cell and using the energy and cell structures of the host cell. For this reason, viruses are said to be obligate parasites.
The stages in viral replication
The virus replication cycle occurs in five main stages:
Attachment The virus attaches to the host cell. This happens when proteins on the virus match up with receptor sites on the host cell wall or membrane. Viruses are often specific to one particular host or even to one type of cell in a host because these cells have suitable receptors.
Entry Sometimes the virus injects its DNA or RNA into the host cell while the virus stays outside. If the virus itself enters the host cell, the protein coat of the virus is digested by the host.
Synthesis The DNA of the host is made inactive. The DNA or RNA of the virus causes the host cell to produce new viral DNA or RNA and new viral proteins. All these new molecules are made using the organelles and energy of the host cell.
Assembly The new viral molecules are used to form new viruses inside the host cell. Each host cell may produce tens of thousands of new viruses.
Release The host cell bursts and releases the new viruses. The damage and death of the host cells causes many of the symptoms of virus infections.
Worked Example: Viral Replication Cycle
Consider a single infected cell:
- Attachment: Virus binds to specific receptors
- Entry: Viral genetic material enters the cell
- Synthesis: Host cell produces 50,000 copies of viral components
- Assembly: These components form 50,000 new viruses
- Release: Cell bursts, releasing all 50,000 viruses to infect other cells
This shows how viral infections can spread so rapidly through tissues.
DNA and RNA viruses
Viruses can be classified based on their genetic material:
DNA viruses:
- Contain DNA as genetic information
- Replicate in the host cell nucleus
- Have lower mutation rates
- Cause diseases like cold sores, chickenpox, and warts
RNA viruses:
- Contain RNA as genetic information
- Replicate in the host cell cytoplasm
- Have higher mutation rates
- Cause diseases including influenza, common cold, polio, COVID-19, AIDS, and various plant diseases
Preventing microbial diseases
Microbial diseases can be prevented by a range of strategies including personal hygiene, food and water sanitation, and private and public health measures. Examples of these methods include:
- Personal hygiene, such as proper handwashing and wearing personal protective equipment (PPE) such as face masks, where appropriate
- Correct handling, cooking and storage of food
- Access to clean water and toilet facilities
- Reduction in the number of disease vectors, such as flies and ticks
- Isolating ourselves when we have an infection that can be passed on
- Ensuring we are vaccinated for all the recommended infections
- Breastfeeding
- Avoiding the misuse of antibiotics in order to reduce the development of antibiotic-resistant bacteria
Prevention is often more effective and less costly than treatment, making these public health measures essential for controlling infectious diseases.
Types of adaptive immunity
Understanding how immunity works helps us prevent and treat infectious diseases. There are different types of adaptive immunity:
Active immunity occurs when antibodies are produced by the person's own body:
- Natural active immunity: pathogens enter the body in the normal way
- Artificial active immunity: pathogens enter the body by injection
Passive immunity occurs when antibodies enter the body by infection:
- Natural passive immunity: antibodies enter the body in a normal way
- Artificial passive immunity: antibodies enter the body by injection
The emergence and spread of infectious diseases
Emerging infectious diseases include those that have recently appeared in a population or have previously existed but are now rapidly increasing in frequency or geographic range. Such diseases affect both plants and animals and can arise in several ways.
Emergence of diseases
Mutations: Microbes evolve (by mutation) to form slightly different forms. Organisms may not be immune to these new forms. This was seen in the Covid-19 pandemic as new variants of the SARS-CoV-2 virus continued to arise.
Antimicrobial resistance (AMR): The overuse and misuse of antibiotics (and other anti-microbial drugs) leads to an increased risk of bacteria and other pathogens that are resistant to antibiotics and other drugs. This leads to increasing numbers of infections that are very difficult to control.
Antimicrobial Resistance Crisis The misuse of antibiotics is creating "superbugs" that are resistant to multiple drugs, making some infections nearly impossible to treat. This is why it's crucial to only use antibiotics when prescribed and to complete the full course.
From animal viruses: Zoonosis is when an infectious disease jumps from a non-human animal to a human. For example, the original SARS-CoV-2 virus is thought to have emerged from bats (and may have been passed on to raccoon dogs and then to humans).
Environmental changes: Factors such as deforestation, climate change and the spread of cities and towns may bring people into contact with wildlife. This may lead to new diseases emerging in human populations. For example, because of climate change, mosquitoes are moving into new regions (and carrying diseases such as malaria with them).
Biological research: Biological research, such as laboratory-designed microorganisms, may be accidentally released or intentionally used to spread disease among people, plants and animals.
Spread of diseases
Several factors affect how infectious diseases spread:
Persistence of the pathogen in a host: The longer a pathogen can survive in a host, the more it can spread.
Immunity level in the population: If a pathogen is common in a population or if the population is immunised (or vaccinated) against the pathogen, many people will have some immunity against it. This will reduce the spread of the disease.
Mobility of the affected population: The more mobile a population is, the more likely a disease is to spread. As populations become more mobile, there is an increasing movement of animals, plants and humans. These movements may be on a local, regional or global scale and can take place very quickly with modern rapid transport methods.
Reproduction number (R₀)
R₀ (pronounced R nought) is a mathematical term that tells how contagious (or spreadable) an infectious disease is.
R₀ predicts the average number of people who will contract a contagious disease from one person with that disease. The R₀ value applies to populations that do not have (or have not had) the infection and who have not been vaccinated for it.
Worked Example: Understanding R₀ Values
If a disease has an R₀ of 6, a person who has the disease will pass it on to an average of 6 other people.
Interpreting R₀ values:
- If R₀ < 1: Each infected person passes the infection on to less than one person → Disease will decline and die out
- If R₀ = 1: Each infected person passes the infection on to one person → Disease remains stable
- If R₀ > 1: Each infected person passes the infection to more than one person → Disease will increase and may cause an epidemic
For comparison: Measles has R₀ ≈ 12-18, while seasonal flu has R₀ ≈ 1.3
Transmission and spread of infectious diseases
Infectious diseases can be transmitted from an infected host to a new host by:
- Tiny particles that hang in the air (and can remain suspended in still air for a long time)
- Droplets in the air (these are heavier and quickly fall to the ground)
- Direct physical contact (touching an infected surface or by sexual contact)
- Being transferred from faeces to the mouth (hence the need to wash our hands)
- Being spread by vectors (a vector is an organism that carries a disease, e.g. a mosquito)
Key Points to Remember:
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Infectious diseases are caused by pathogens (bacteria, viruses, fungi, parasites) that invade and multiply in the body
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Different treatments are needed for different pathogens - antibiotics for bacteria, antivirals for viruses, and antifungals for fungi
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Virus replication follows five stages: attachment, entry, synthesis, assembly, and release - viruses are obligate parasites that need host cells to reproduce
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Disease emergence can occur through mutations, antimicrobial resistance, animal-to-human transmission, environmental changes, and biological research accidents
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The R₀ value tells us how contagious a disease is - less than 1 means decline, equal to 1 means stable, more than 1 means epidemic potential