Biology Topic 3: Infection, Response, and Immune Defenses

Communicable Diseases and How They Spread

Pathogens are tiny microorganisms like bacteria, viruses, protists and fungi that cause disease in humans, plants and animals. Unlike non-communicable diseases, these nasty bugs can jump from person to person, making them properly contagious.

These disease-causing microbes spread in three main ways. Airborne transmission happens when infected people cough or sneeze, sending droplets flying through the air - this is how influenza and measles get around. Contaminated food and water spread diseases like cholera and salmonella, whilst direct contact with infected surfaces can give you athlete's foot.

The good news? You can stop diseases spreading by washing your hands regularly, killing disease-carrying insects (vectors) with insecticides, getting vaccinated, and isolating infected people. Vaccination is particularly brilliant because if you can't catch a disease, you definitely can't pass it on to others.

Remember: Good hygiene is your first line of defence against communicable diseases!

Viral Diseases You Should Know About

Viruses aren't technically alive but they're still classed as organisms - and they're absolutely tiny, about 10,000 times smaller than animal cells. The sneaky thing about viruses is they can't reproduce by themselves, so they hijack your cells to make copies.

Measles spreads through airborne droplets and causes a distinctive red rash plus fever. It used to be deadly but it's rare in the UK now thanks to childhood vaccinations. HIV is much more serious - it spreads through sexual contact or sharing contaminated needles and blood.

HIV starts with flu-like symptoms that seem to disappear, but the virus keeps damaging your immune system until it becomes AIDS. Your body becomes so weak it can't fight off other infections or cancers. Luckily, antiretroviral drugs now stop the virus replicating, meaning HIV doesn't have to be a death sentence.

Tobacco Mosaic Virus (TMV) affects plants like tobacco and tomatoes, creating a distinctive mosaic pattern on leaves that stops photosynthesis and stunts growth.

Key point: Many viral diseases that used to kill people are now preventable through vaccination!

Bacterial and Other Diseases

Bacteria are single-celled organisms that reproduce rapidly inside your body, producing toxins that damage your cells and tissues. Unlike viruses, bacteria are proper living organisms that can multiply all by themselves.

Salmonella comes from infected chickens and causes fever, stomach cramps, diarrhoea and vomiting. Most UK chickens are vaccinated against it now, so it's becoming rarer. Gonorrhea is a sexually transmitted bacterial infection causing painful urination and thick yellow-green discharge.

Fungi like the organism causing rose black spot spread through water and wind, making plant leaves turn yellow and drop off. This reduces the plant's ability to photosynthesise properly. You can treat fungal diseases by removing infected leaves or spraying with fungicides.

Protists are mostly single-celled organisms, with some acting as parasites. They're often transported by vectors (like mosquitoes) that carry the disease between hosts without getting ill themselves. You can control protist diseases by destroying vector breeding sites or using insecticides and repellents.

Treatment tip: Bacterial infections can be treated with antibiotics, but some bacteria are becoming resistant to common antibiotics like penicillin.

Your Body's Defence System

Your body has brilliant physical and chemical barriers to keep pathogens out. Your skin acts as a protective wall and secretes antimicrobial substances that kill germs on contact. When pathogens try to enter through your nose and mouth, you've got backup defences ready.

Your nose has tiny hairs and mucus that trap pathogens, whilst your trachea is lined with cilia (finger-like structures) that sweep particles back to your throat to be swallowed. Any pathogens that make it to your stomach get destroyed by hydrochloric acid.

If pathogens break through these barriers, your immune system springs into action. White blood cells patrol your body and have three key jobs: phagocytosis (literally eating pathogens), producing antitoxins to neutralise harmful toxins, and making antibodies - specific proteins that target particular antigens.

The clever bit is that your immune system remembers each pathogen, so if the same bug tries to infect you again, your body can respond much faster and more effectively.

Amazing fact: Your white blood cells can actually engulf and digest whole pathogens through phagocytosis!

Vaccinations and Medicine

Vaccines contain dead or weakened versions of pathogens that can't cause disease but still trigger your immune system to build up immunity. It's like giving your body a practice run so it knows exactly what to do if it meets the real pathogen later.

Vaccination has brilliant benefits: it protects you from diseases, helps control common illnesses like polio and measles, prevents outbreaks, and creates herd immunity where enough people are protected to stop diseases spreading. However, vaccines don't always give complete immunity and some people have bad reactions.

There are two types of medicines: those that just relieve symptoms (like aspirin and paracetamol for pain, or cough medicine) and those that actually treat the disease. Antibiotics fight bacterial infections but they only work on specific bacteria, and some bacteria are becoming resistant to them.

Many medicines originally came from plants - aspirin from willow tree bark, digitalis from foxgloves, and penicillin from fungus. Scientists can either use these natural compounds directly or modify them in labs to make them more effective.

Important: Antibiotics only work against bacterial infections, not viral ones like colds or flu!

Developing and Testing New Medicines

Before any new medicine reaches patients, it goes through rigorous testing to check three crucial things: efficacy (how well it works), toxicity (how harmful it is), and dosage (how much to give for maximum benefit with minimum side effects).

Stage 1 involves testing on human cells and tissues grown in labs - it's cheap and quick but doesn't show how the drug affects whole organisms. Stage 2 uses live animals like mice and rabbits because they're mammals like us, giving better insight into efficacy and toxicity.

Stage 3 involves human trials, starting with healthy volunteers on low doses that gradually increase whilst doctors monitor for side effects. Once the maximum safe dose is found, the drug is tested on patients with the actual condition to find the optimum balance between effectiveness and safety.

To ensure fair results, many trials use placebos (fake drugs that do nothing) and double-blind testing where neither patients nor doctors know who's getting the real treatment. This prevents bias from affecting the results.

Safety first: It can take many years to properly test a new medicine before it's approved for public use.

Monoclonal Antibodies and Pregnancy Tests

Monoclonal antibodies are identical antibodies produced from a single clone of cells in laboratories. Scientists make them by combining B-lymphocytes (white blood cells that normally make antibodies) with fast-dividing tumour cells to create hybridomas.

These hybridomas divide rapidly in petri dishes, producing loads of identical antibodies that can be collected and purified. To get the right B-lymphocytes in the first place, scientists inject mice with the specific antigen they want the antibody to target.

One brilliant use of monoclonal antibodies is in pregnancy tests. Pregnant women produce a hormone called HCG that appears in their urine. The pregnancy test strip contains blue beads covered with HCG-specific antibodies.

If you're not pregnant, the beads just wash along the test strip and nothing happens. But if you are pregnant, the HCG in your urine binds to the antibodies on the blue beads, which then stick to fixed antibodies further along the strip, making it turn blue.

Cool application: Monoclonal antibodies are also being developed to treat cancer by targeting specific cancer cells!

Plant Diseases and Natural Defences

Plants get diseases from microorganisms like fungi, viruses and bacteria, plus larger pests like insects. They can also suffer from deficiency diseases when they don't get enough essential nutrients like nitrates (needed for proteins and growth) or magnesium ions (essential for chlorophyll and photosynthesis).

Signs of plant disease include abnormal growths, areas of decay, malformed stems and leaves, or spots of discolouration. Sometimes you can actually see the culprits like aphids or spider mites crawling about.

To diagnose plant diseases, start with basic observations and check gardening manuals or websites. If that doesn't work, send samples to plant pathologists who can do detailed testing, look for unique antigens, or even run DNA tests to identify pathogen DNA.

Plants have evolved three main types of defences. Physical barriers like waxy cuticles, cellulose cell walls and bark prevent pathogen entry. Chemical defences include antimicrobial substances and poisons that kill or deter attackers. Mechanical defences like thorns, hairs, or leaves that curl and droop make plants harder to attack.

Plant power: Just like humans, plants have multiple layers of defence to protect themselves from disease!