Human Health and Diseases: Causes, Immunity and Prevention for NEET by PW

Human Health and Diseases has a long list of pathogens, symptoms, vaccines and drugs, and many NEET students struggle to keep them apart. Typhoid gets mixed with pneumonia, dengue with chikungunya, and active immunity with passive immunity. One small mix-up can cost you a valuable mark in the exam.

The details below follow a simple order: what health means, then diseases by pathogen type, immunity, AIDS, cancer and substance abuse. Each part uses plain words and short points, so revision is faster and easier. PW’s online Biology classes can also help you practise the same chapter with notes and tests.

What is Health and What is a Disease?

The World Health Organisation (WHO) says health is more than having no disease or being physically fit. It means a state of complete physical, mental and social well-being. Knowing this definition first helps you understand why diseases are grouped the way they are.

Long ago, Hippocrates and the Indian Ayurveda system believed that health depended on a balance of body fluids called humours: blood, yellow bile, black bile and phlegm. Too much black bile was linked to a “hot personality” and fever. This is called the good humour hypothesis. William Harvey, who discovered blood circulation, tested it by measuring body temperature and found it normal in people with more black bile. So the idea was disproved.

Three things affect health:

• Genetic disorders: Problems present from birth.

• Lifestyle: Diet, exercise and hygiene.

• Infections: Contact with disease-causing agents.

A disease is any condition that harms the normal working of the body or its organ systems. Diseases are of two main types:

• Congenital diseases: Present from birth, such as Down syndrome and sickle cell anaemia.

• Acquired diseases: Gained during life. These are of two kinds. Communicable (infectious) diseases spread easily and are caused by viruses, bacteria, fungi, helminths or protozoa. Non-communicable (non-infectious) diseases do not spread from person to person, such as cancer and allergies.

Bacterial Diseases: Typhoid and Pneumonia

Bacterial diseases are caused by harmful bacteria that can affect different organs of the body. They spread through contaminated food and water, air droplets or direct contact. Typhoid and pneumonia are two important bacterial diseases, along with plague and others.

• Typhoid

Typhoid is caused by the bacterium Salmonella typhi and mainly affects the small intestine. It spreads through contaminated food and water. Mary Mallon, known as Typhoid Mary, spread it as a carrier while working as a cook.

The symptoms are a steady high fever (39 to 40°C), loss of appetite, weakness, stomach pain, headache and constipation. In severe cases, the intestine can be perforated, which can be fatal. The Widal test is used to diagnose typhoid.

• Pneumonia

Pneumonia is caused by Streptococcus pneumoniae and Haemophilus influenzae. It infects the lungs, where the alveoli fill with fluid. It spreads through droplets from coughing and sneezing and through contaminated objects.

The symptoms are fever, headache, cough, chills and trouble breathing. In severe cases, the lips and fingernails turn greyish or bluish because of low oxygen.

Do not confuse pneumonia with the common cold. Pneumonia affects the lungs and can be life-threatening. The common cold affects only the nasal passages, is caused by viruses and is usually mild.

Other bacterial diseases to remember are plague (Yersinia pestis, also called the Black Death), diphtheria (Corynebacterium diphtheriae) and dysentery (often caused by Shigella).

Viral Diseases: Common Cold, Dengue and Chikungunya

Viruses rely entirely on host cellular machinery to replicate, causing symptoms ranging from localised respiratory distress to severe systemic complications. Below is an overview of the most common viral infections, their transmission vectors, and key distinguishing symptoms. 

• Common Cold

The common cold is caused by rhinoviruses. It infects the upper respiratory tract and the nasal passages, but it does not reach the lungs. It spreads through infected nasal droplets and contaminated objects. The symptoms are a runny or blocked nose, sore throat, cough, headache, fever and tiredness. It usually lasts 3 to 7 days.

• Dengue

Dengue is caused by the dengue virus, an RNA virus and an arbovirus. The female Aedes mosquito spreads it. The symptoms are high fever, severe headache, pain behind the eyes, joint pain, rash and a fall in blood platelets. Severe dengue can cause internal bleeding.

• Chikungunya

Chikungunya is caused by the chikungunya virus, which is also an RNA virus and an arbovirus. It is also spread by the female Aedes mosquito. The symptoms are high fever, headache, body pain, nausea and rash. Its key sign is severe and long-lasting joint pain.

Both diseases share the same vector, so exam questions often ask how to tell them apart. Dengue is marked by pain behind the eyes, a large platelet drop and a risk of internal bleeding. Chikungunya is marked by severe, often persistent joint pain.

How Can Mosquito-Borne Diseases Be Prevented?

The best way to stop these diseases is to control mosquitoes.

• Avoid stagnant water: Do not let water collect in coolers and flower pots.

• Wear protective clothing: Full-sleeved clothes lower the risk of bites.

• Control larvae: Spray kerosene oil on water bodies, or add Gambusia fish, which eat mosquito larvae.

Helminthic and Fungal Diseases

Parasitic worms and fungal agents exploit body tissues and mucosal surfaces, causing chronic inflammation and mechanical blockages. The following points summarise the primary parasitic helminths and cutaneous fungi relevant to the syllabus. 

• Ascariasis

Helminthic diseases are caused by aschelminthes. Ascariasis is caused by Ascaris lumbricoides, the roundworm, which lives in the small intestine. It spreads when a person eats food or drinks water containing the worm’s eggs, which pass out in faeces. Remember the symptoms with the tip MIA: Muscular pain, Internal bleeding, Anaemia and blockage of the intestine.

• Filariasis

Filariasis, also called elephantiasis, is caused by the filarial worms Wuchereria bancrofti and Wuchereria malayi. They live in the lymphatic vessels and cause swelling and blockage. The bite of an infected female Culex mosquito spreads the disease. The legs, scrotum or mammary glands swell, and the skin becomes thick, dry and wrinkled. This is why it is called elephantiasis.

• Ringworm

Ringworm is a fungal disease caused by Microsporum, Epidermophyton and Trichophyton. It affects the skin, nails and scalp, and it grows best in warm, moist places such as skin folds. It spreads through direct contact, shared towels and clothes, or soil. The signs are dry, scaly patches with strong itching.

Protozoal Diseases: Amoebiasis and Malaria

Protozoan parasites display complex life cycles, often requiring multiple hosts to complete sexual and asexual reproduction stages. Here is a breakdown of intestinal amoebiasis and the multi-stage developmental cycle of Plasmodium. 

• Amoebiasis

Amoebiasis, or amoebic dysentery, is caused by the protozoan Entamoeba histolytica and affects the large intestine. It spreads through contaminated food and water. Houseflies act as mechanical carriers, which means they carry the parasite without hosting its life cycle. The symptoms are stomach pain, cramps, constipation, and stools with a lot of mucus and blood clots.

• Malaria

Malaria is caused by Plasmodium, a protozoan. Plasmodium falciparum causes the deadliest type, called cerebral or malignant malaria. The bite of an infected female Anopheles mosquito spreads it.

Plasmodium needs two hosts, so it is digenetic. The female Anopheles mosquito is the primary host, where the sexual part of the life cycle happens. The human is the secondary host, where the parasite reproduces asexually.

In humans:

1. The mosquito injects sporozoites, the infective stage, into the blood.

2. The sporozoites reach the liver, enter liver cells and multiply by asexual reproduction. The liver cells burst and release the parasites.

3. The parasites enter red blood cells (RBCs), multiply and use up haemoglobin. The heme part turns into a toxic substance called hemozoin. When RBCs burst, hemozoin enters the blood and causes chills and fever. Repeated bursting leads to anaemia.

4. Some parasites become gametocytes in the RBCs, but they cannot mature further in humans.

In the mosquito:

1. The mosquito takes in gametocytes when it bites an infected person.

2. In the mosquito’s midgut, the gametocytes become gametes, which fuse to form a zygote. The zygote develops into sporozoites.

3. The sporozoites move to the salivary glands, ready to infect another person.

How Does Immunity Work?

Immunity is the body’s defence against pathogens. It is of two types.

• Innate immunity: Present from birth. It is non-specific, treats all pathogens the same way and has no memory.

• Acquired immunity: Developed during life. It is highly specific and remembers past pathogens, so it gives a faster and stronger response next time.

The first contact with a pathogen gives the primary response, which is slow and weak. A second contact gives the secondary, or anamnestic, response, which is much faster and stronger because of memory cells. Vaccination works on this idea.

Antibodies

An antibody (immunoglobulin) is written as H2L2. It has four polypeptide chains: two heavy and two light, joined by disulphide bonds. It has two antigen-binding sites at the N-terminal ends. Each chain has a variable region, which decides what the antibody can bind to, and a constant region.

There are five types of antibodies:

• IgG: The most common, at 75 to 80%. It crosses the placenta and gives passive immunity to the baby.

• IgA: Found in secretions such as saliva, mucus and colostrum.

• IgM: The largest antibody, a pentamer. It is mainly made in the primary response.

• IgE: Present in low amounts and linked to allergies. It binds to mast cells, which release histamine, serotonin and heparin.

• IgD: The least common. It sits on the surface of B lymphocytes and works as an antigen receptor.

T-Cells and Graft Rejection

Cell-mediated immunity depends on T lymphocytes. Helper T cells activate other immune cells, and their number falls in AIDS. Cytotoxic (killer) T cells destroy infected or abnormal cells. Suppressor T cells control the immune response, and memory T cells give a faster second response.

Cell-mediated immunity also causes graft rejection after an organ transplant. The body sees mismatched MHC proteins on the donor organ as non-self. Doctors give immunosuppressants to stop rejection.

Active and Passive Immunity

In active immunity, the body makes its own antibodies after meeting an antigen. It is slow to start but long-lasting, and it forms memory cells. It can be natural, after an infection, or artificial, after vaccination with weakened or killed pathogens or their parts.

In passive immunity, the body gets ready-made antibodies from outside. Protection is immediate but short-lived, and there are no memory cells. It can be natural, such as IgG crossing the placenta and IgA in colostrum, or artificial, such as antivenom and antitetanus serum.

Vaccination, Allergy, Autoimmunity and Lymphoid Organs

Immunological balance depends on specialised lymphoid tissues that educate immune cells and prevent aberrant responses. This section details how protective immunity is induced through vaccines and what occurs when self-tolerance or allergen control breaks down. 

• Vaccination and Immunisation

Vaccination means giving a vaccine, and immunisation is the immunity that results. A vaccine contains weakened or killed pathogens, toxoids or recombinant proteins, such as the hepatitis B vaccine made in yeast. It makes memory cells without causing the disease, so the body is ready for a quick secondary response.

• Allergy

An allergy is an exaggerated immune response to harmless substances called allergens, such as pollen, animal hair, dust mites and some foods. IgE levels rise, and mast cells and basophils release histamine, heparin and serotonin. This causes sneezing, watery eyes, a runny nose and breathing trouble. Antihistamines, adrenaline or cortisol are used for treatment. Hay fever and asthma are common examples.

• Autoimmunity

In autoimmunity, the immune system cannot tell self from non-self and attacks the body’s own cells. Examples are myasthenia gravis, rheumatoid arthritis and exophthalmic goitre.

• Lymphoid Organs

Lymphoid organs are where lymphocytes form, mature and multiply.

• Primary lymphoid organs: Where lymphocytes are made and mature. These are the bone marrow, which matures B cells and makes all blood cells, and the thymus, which matures T cells.

• Secondary lymphoid organs: Where lymphocytes meet antigens. These include the spleen, tonsils, appendix, Peyer’s patches in the small intestine, lymph nodes and MALT (mucosa-associated lymphoid tissue). MALT makes up about 50% of lymphoid tissue and lies along the respiratory, digestive and reproductive tracts.

The spleen filters blood and is called the graveyard of RBCs, while lymph nodes filter lymph.

AIDS

AIDS (Acquired Immunodeficiency Syndrome) is caused by HIV, the human immunodeficiency virus, which is a retrovirus with RNA. World AIDS Day is on 1 December.

HIV spreads in four ways:

• From an infected mother to her baby.

• Through infected blood transfusions.

• By sharing infected needles, which is common among drug abusers.

• Through unprotected sex.

It does not spread through casual touch or by sharing food.

HIV has an outer glyco-lipid layer with GP120 pins, which help it attach to host cells. Inside are the P17 and P24 capsid layers, two strands of single-stranded RNA, and the enzymes reverse transcriptase and integrase.

HIV mainly infects macrophages, which act as a factory, and helper T cells. GP120 binds to CD4 receptors on the host cell. Reverse transcriptase turns the viral RNA into DNA, and integrase adds it to the host DNA. The host cell then makes new viruses. Helper T cells keep getting destroyed, and their count can fall from about 1200 to about 200 per mm³. The body then becomes open to opportunistic infections.

Symptoms can take 5 to 10 years to appear. ELISA is the main test, and PCR can also detect HIV. Antiretroviral drugs only partly work. They extend life but do not cure the disease.

Prevention is the key: avoid unknown sexual partners, do not share needles, make sure blood is safe, and ask a doctor for advice. NACO and WHO run public education programmes.

Cancer

Cancer is uncontrolled cell division that forms a lump called a tumour or neoplasm. Cancer cells lose contact inhibition and are called immortal.

Tumours are of two kinds. Benign tumours stay in one place, are often enclosed in a covering and do not spread. Malignant tumours are masses of neoplastic cells that can move. They show metastasis, which means cells break off and form new tumours elsewhere. This is the most feared property of cancer.

Cancer-causing agents are called carcinogens:

• Physical: Ionizing radiation such as X-rays and gamma rays, and non-ionizing radiation such as UV rays.

• Chemical: Substances found in tobacco.

• Biological: Viruses such as the human papilloma virus and hepatitis B virus.

Carcinoma, which affects epithelial tissue, is the most common cancer at about 85%. Normal genes that control cell growth are called proto-oncogenes. Carcinogens can turn them into oncogenes, which cause cancer.

Doctors diagnose cancer using histopathology and biopsy, imaging such as CT scan and MRI, blood and bone marrow tests for leukaemia, and antibody tests. Genetic testing can show a person’s risk.

Treatment often combines several methods:

• Surgery: Removing the tumour.

• Radiotherapy: Using radiation to destroy cancer cells.

• Chemotherapy: Using strong drugs. Side effects include weight loss, hair loss and anaemia.

• Immunotherapy: Waking the patient’s own immune system, including natural killer cells. Alpha interferons are used as biological response modifiers.

Drug and Alcohol Abuse

Addiction is a habit that grows into dependence. It is a serious concern, especially among teenagers. The main groups of drugs are these:

• Tranquillisers (depressants): Slow down the central nervous system (CNS) and cause calm and sleep. They treat anxiety and insomnia. Examples are benzodiazepines and barbiturates.

• Opiate narcotics (depressants): Reduce pain and act as sedatives. Morphine comes from the opium poppy (Papaver somniferum) and is a strong painkiller. Heroin, or smack, is made by diacetylation of morphine. It is white, crystalline, bitter and odourless.

• Stimulants: Raise activity, alertness and energy. Cocaine (coke, crack) comes from Erythroxylum coca of South America. It blocks dopamine transport, which causes euphoria, and an overdose can cause hallucinations. Amphetamines are synthetic stimulants that sportspersons misuse.

• Hallucinogens: Cause hallucinations and can lead to psychosis. Cannabinoids come from Cannabis sativa and include hashish, charas, ganja and marijuana. They affect the brain and the heart. Others are dhatura, Atropa belladonna and LSD, which comes from the fungus Claviceps purpurea.

• Anabolic steroids: Synthetic forms of testosterone that sportspersons misuse to build muscle.

Tobacco contains nicotine. It is linked to emphysema, bronchitis, gastric ulcers, lung cancer, oral cancer and coronary artery disease. Tobacco smoke raises carbon monoxide (CO) in the blood. CO binds to haemoglobin 200 to 250 times more strongly than oxygen does, so less oxyhaemoglobin forms and the body gets too little oxygen. Tobacco also makes the adrenal gland release epinephrine and norepinephrine, which raise the heart rate and cardiac output.

Anabolic steroid side effects:

• In both males and females: Aggressiveness, mood swings and depression.

• In females: Masculinisation, irregular periods, extra hair on the face and body, an enlarged clitoris and a deeper voice.

• In males: Smaller testicles, lower sperm production, infertility, possible kidney or liver problems, breast enlargement, early baldness and an enlarged prostate.

Alcohol is absorbed mainly through the stomach and acts as a CNS depressant. Too much alcohol damages the liver, harms the heart and blood vessels and causes cirrhosis, in which fibrous tissue replaces liver tissue. Drinking during pregnancy harms the baby’s growth.

When a person suddenly stops using drugs or alcohol, they can get withdrawal symptoms such as aggressiveness, shaking, sweating, nausea and destructive behaviour.

Substance abuse can be prevented by:

• Saying no to peer pressure.

• Getting education and counselling.

• Seeking support from parents and friends.

• Learning the danger signs early.

• Taking professional and medical help.

Once you can link each disease to its cause, spread and prevention, the chapter becomes much easier to revise. For regular practice with live classes and tests, you can explore PW NEET online coaching.

Related Resources

• NEET Biology syllabus

• Class 12 Biology notes

• Reproduction in Organisms

• Human Reproduction

• NEET Biology important chapters

• NEET previous year question papers