Human Health And Diseases: Complete NEET Preparation

Human health depends on more than the absence of illness. Genetic disorders, daily lifestyle, physical activity and exposure to infections all shape how well the body functions. Diseases can arise from biological agents such as bacteria, viruses, protozoa and helminths, or from internal changes like uncontrolled cell division and exaggerated immune responses. Depending on the cause, they may affect organs such as the intestine, lungs, liver, blood or immune system.

This chapter is high-scoring in NEET because the questions are direct and fact-based. You need to recall causative agents, sites of infection, modes of transmission, symptoms and confirmatory tests. You also need to separate look-alike topics, such as pneumonia and the common cold, or HIV and AIDS. Immunity types, antibody classes, cancer, allergy and drug abuse complete the chapter.

Meaning And Factors Affecting Health

According to the World Health Organization (WHO), health is the complete physical, mental, and social well-being of a person. It does not simply mean the absence of disease.

The earlier Good Humour Hypothesis stated that health depended on the balance of body fluids: blood, yellow bile, black bile, and phlegm. It was later disproved. William Harvey’s work on blood circulation showed that blood continuously circulates through a closed system, with the heart acting as a pump.

Major factors affecting health include:

• Genetic disorders, which may affect a person from birth

• Lifestyle, including exercise, yoga, and regular physical activity

• Infections caused by agents such as bacteria and fungi

Classification Of Diseases

Diseases are broadly classified into:

Acquired diseases may be:

• Communicable or Infectious Diseases: Spread from an infected person to a healthy person

• Non-Communicable or Non-Infectious Diseases: Do not spread between people

Cancer and allergy are non-communicable diseases.

Important Bacterial Diseases

Bacterial diseases are caused by harmful bacteria that enter the body and affect different organs. Some common bacterial diseases spread through contaminated food and water, while others spread through different routes. 

yphoid

• Causative Agent: Salmonella typhi

• Site: Small intestine

• Transmission: Contaminated food and water

• Confirmatory Test: Widal Test

Symptoms include sustained high fever of about 39 to 40 degrees Celsius, loss of appetite, weakness, weight loss, stomach pain, constipation, and headache. Loss of appetite reduces food intake, which leads to weakness and weight loss. Severe cases may cause intestinal perforation or death.

Typhoid Mary, or Mary Mallon, was a cook and a typhoid carrier who spread the disease through contaminated food.

Pneumonia

Pneumonia is a bacterial infection of the lungs, especially the alveoli. Its causative agents include Streptococcus pneumoniae and Haemophilus influenzae.

Fluid accumulates in the alveoli and interferes with gaseous exchange. Transmission occurs through infected droplets and contaminated objects, called fomite-borne transmission.

Symptoms include fever, headache, cough, chills, difficulty in breathing, and fluid in the alveoli. Severe oxygen deficiency may turn the lips and fingernails greyish-blue.

Other bacterial diseases include:

• Plague: Yersinia pestis, also called Black Death

• Diphtheria: Corynebacterium diphtheriae

• Dysentery: Shigella

Important Viral Diseases

Viral diseases are caused by viruses that infect different parts of the human body. Some viral infections affect the respiratory system, while others spread through insect bites and can cause fever, body pain, and other symptoms.

Common Cold

The common cold is caused by rhinoviruses and affects the upper respiratory tract and nasal passages. It spreads through infected droplets and contaminated objects.

Symptoms include nasal discharge, running nose, congestion, fatigue, headache, fever, sore throat, throat irritation, and hoarseness. Symptoms usually last for 3 to 7 days. The common cold is not an infection of the lungs.

Dengue And Chikungunya

Both are RNA virus infections spread by the female Aedes mosquito. They are also called arboviral diseases because they are transmitted by arthropod vectors.

Dengue may cause fever, headache, body pain, joint pain, rashes, pain around the eyeballs, and a reduced blood platelet count. Severe dengue may cause internal bleeding.

Chikungunya has similar symptoms, but its most important distinguishing feature is severe joint pain, which may cause difficulty in standing and walking.

Prevention Of Mosquito-Borne Diseases

• Avoid stagnant water

• Change cooler water regularly

• Wear full-length clothing

• Use mosquito repellents

• Add kerosene oil to stagnant water where suitable

• Introduce Gambusia fish, which feed on mosquito larvae

Helminthic And Protozoal Diseases

Helminthic and protozoal diseases are caused by parasitic worms and single-celled organisms, respectively. These infections can affect different organs and spread through contaminated food, water, or insect bites, depending on the causative agent.

Ascariasis

Ascariasis is caused by Ascaris, a helminth of phylum Aschelminthes. Adult worms live in the small intestine. Eggs pass out through faecal matter and may contaminate soil, crops, food, and water.

Symptoms include muscular pain, internal bleeding, anaemia, and intestinal blockage. 

Filariasis

Filariasis, also called elephantiasis, is caused by Wuchereria bancrofti and Wuchereria malayi. It spreads through the bite of an infected female Culex mosquito.

The worms live in the lymphatic vessels, causing inflammation, blockage of lymph flow, accumulation of lymph, and swelling. Legs and the scrotum are commonly affected. The skin becomes thick, dry, and wrinkled, giving the condition its name.

Amoebiasis

Amoebiasis, or amoebic dysentery, is caused by Entamoeba histolytica and affects the large intestine. It spreads through contaminated food and water. The housefly acts as a mechanical carrier.

Symptoms include abdominal pain, cramps, constipation, excessive mucus, and blood clots in stool. 

Malaria

Malaria is caused by Plasmodium and transmitted by the infected female Anopheles mosquito. The mosquito is both a vector and the primary host. Humans are the secondary host.

• Sexual reproduction occurs in the mosquito.

• Asexual reproduction occurs in humans.

• Sporozoites enter humans through mosquito saliva.

• They first infect liver cells, or hepatocytes.

• They then enter red blood cells and multiply.

• RBC rupture releases haemozoin, which is linked to chills and fever.

• Repeated RBC rupture may cause anaemia.

• Gametocytes enter the mosquito during a blood meal.

• Gametes fuse in the mosquito midgut, eventually forming sporozoites.

Plasmodium falciparum causes the most dangerous form, called cerebral malaria or malignant malaria.

Immunity

Innate Immunity is present from birth, non-specific, and has no immunological memory. Its barriers are:

• Physical Barriers: Skin and mucus linings

• Physiological Barriers: HCl in the stomach and lysozyme in tears and saliva

• Cellular Barriers: PMNLs, monocytes, macrophages, and natural killer cells

• Cytokine Barriers: Interferons released by virus-infected cells

Acquired Immunity develops during life, is highly specific, and has memory. Its secondary response is faster and stronger.

• Humoral Immunity: B lymphocytes produce antibodies

• Cell-Mediated Immunity: T lymphocytes act against infected or abnormal cells

Antibodies And Immunoglobulins

A standard antibody has two heavy chains and two light chains, represented as H2L2. The chains are joined by disulfide bonds. Its variable region binds the antigen through a paratope.

AIDS And HIV

AIDS means Acquired Immunodeficiency Syndrome. It is caused by HIV, a retrovirus containing RNA and reverse transcriptase.

HIV spreads through infected blood, infected needles, unprotected sexual contact, and transmission from an infected mother to her baby. It does not spread through ordinary social contact or sharing meals.

HIV targets macrophages and T-helper cells. It uses GP120 to attach to CD4 receptors, converts RNA into DNA using reverse transcriptase, and integrates viral DNA into the host genome using integrase. Destruction of T-helper cells causes opportunistic infections.

• Diagnosis: ELISA

• Confirmatory Test: Western blotting

• Prevention: Safe blood, disposable needles, condoms, safe sexual practices, and education

• Treatment: Antiretroviral drugs are partially effective but do not provide a complete cure

Cancer And Allergy

Cancer involves uncontrolled cell division, loss of contact inhibition, immortality, and tumour formation. Malignant tumours show metastasis, while benign tumours remain localised.

Carcinogens include X-rays, gamma rays, ultraviolet radiation, tobacco chemicals, HPV, and hepatitis B virus. Carcinoma is cancer of epithelial tissue.

Diagnosis may involve biopsy, histopathology, CT scan, MRI, blood tests, and bone marrow tests. Treatment may include surgery, radiotherapy, chemotherapy, and immunotherapy.

An allergy is an exaggerated immune response. IgE, mast cells, and histamine are central to allergic reactions. Common symptoms include sneezing, watery eyes, runny nose, rashes, and breathing difficulty.

Drugs, Alcohol, And Prevention

Drugs and alcohol can affect the brain, nervous system, behaviour, and overall health. Different substances produce different effects on the body, and their misuse can lead to serious health problems and dependence.

• Tranquillisers: Depress the central nervous system; examples include benzodiazepines and barbiturates.

• Opiate Narcotics: Morphine reduces pain; heroin is produced by diacetylation of morphine.

• Stimulants: Cocaine increases dopamine activity; amphetamines increase alertness.

• Cannabinoids: Obtained from Cannabis sativa and cause hallucinations.

• LSD: A potent hallucinogen obtained from Claviceps purpurea.

• Anabolic Steroids: Testosterone derivatives misused for muscle growth and performance.

Tobacco smoke increases carbon monoxide, which has 200 to 250 times greater affinity for haemoglobin than oxygen. Alcohol depresses the central nervous system and may cause liver cirrhosis.

Stopping addictive substances suddenly may cause withdrawal syndrome, including sweating, shakiness, nausea, aggressiveness, and destructive behaviour. Prevention depends on education, counselling, avoiding peer pressure, and professional medical help.

What Common Mistakes Should You Avoid In Human Health And Diseases?

Most marks in this chapter are lost to look-alike facts, not hard concepts. Questions often swap one causative agent, vector, site or antibody for another, so check these points during revision.

• Pneumonia infects the lungs and alveoli, while the common cold is a viral infection of the nose and upper respiratory tract.

• Malaria is spread by the female Anopheles mosquito, dengue and chikungunya by the female Aedes mosquito, and filariasis by the female Culex mosquito.

• Typhoid (Salmonella typhi) affects the small intestine and is confirmed by the Widal test, while amoebiasis (Entamoeba histolytica) affects the large intestine.

• HIV is the virus, and AIDS is the syndrome it causes, so do not use the terms interchangeably. ELISA is the screening test, and Western blotting is the confirmatory test.

• IgG crosses the placenta, IgA is found in saliva, mucus and colostrum, IgM is a pentamer, and IgE rises in allergy.

Human Health and Diseases rewards clear, structured revision: learn each disease by its cause, site, transmission, symptoms, tests, and prevention, and keep immunity, HIV, cancer, allergy and drug abuse concepts distinct. Revise the tables and memory tips regularly, and practise PYQs so these facts stay sharp for NEET 2026.

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