Human Health and Disease

A pathogen is a disease-causing biological agent. A parasite lives in or on a host and derives benefit at host cost; many parasites are pathogens, but pathogen and parasite are not synonyms.

Typhoid

Pathogen: Salmonella typhi. Transmission: contaminated food/water through the faecal-oral route. Primary relationship: intestinal entry followed by systemic spread through blood.

Pneumonia

Representative bacterial pathogens: Streptococcus pneumoniae and Haemophilus influenzae. Transmission: droplets/aerosols and contaminated shared items. Affected structure: lung alveoli.

Common cold

Pathogen group: rhinoviruses. Spread: respiratory droplets and contaminated objects. Affected region: nose and upper respiratory passages, not the alveoli.

Malaria

Pathogen: Plasmodium species, including P. vivax, P. malariae and P. falciparum. Vector: infected female Anopheles mosquito. Human sequence: sporozoites enter, then liver multiplication, then red-blood-cell phase. Vector and pathogen must not be swapped.

Amoebiasis

Pathogen: Entamoeba histolytica. Transmission: contaminated food/water; houseflies can act as mechanical carriers. Primary site: large intestine.

Ascariasis

Pathogen: Ascaris lumbricoides. Route: ingestion of infective eggs through contaminated water/vegetables/food/soil context. Primary relationship: intestinal helminth infection.

Filariasis

Pathogens: Wuchereria bancrofti and W. malayi. Transmission: female mosquito vectors. Main relationship: chronic involvement of lymphatic vessels.

Ringworm

Fungal genera: Microsporum, Trichophyton, Epidermophyton. Spread: contact with infected material, soil or shared personal items. Affected structures: skin, nails and scalp.

Dengue

Officially named in the 2026 syllabus. Causative category: dengue virus. Vector relationship: Aedes aegypti is a principal vector documented by Government of India NCVBDC. This is kept as a pathogen-vector recall relationship, not clinical guidance.

Chikungunya

Officially named in the 2026 syllabus. Causative category: chikungunya virus, transmitted through infected Aedes mosquitoes in official Government of India disease-control material. No symptom-based diagnosis is provided.

Innate immunity is present at birth and responds non-specifically. NCERT organises it through physical, physiological, cellular and cytokine-related barriers.

Acquired immunity is antigen-specific and develops memory.

• Primary response: first exposure, slower/lower initial response.
• Secondary response: memory response, faster/stronger.
• Humoral immunity: antibody-mediated B-cell relationship.
• Cell-mediated immunity: T-cell relationship.

• Active: host generates an immune response after antigen exposure.
• Passive: ready-made antibodies are received.

An antibody is an immunoglobulin produced by plasma-cell descendants of activated B lymphocytes.

Vaccination exposes the immune system to antigenic material in a controlled way so that immunological memory can be generated. This is a syllabus concept, not an individual vaccine recommendation.

HIV is a retrovirus. At NCERT depth: the virus enters macrophages and helper T lymphocytes; viral replication progressively damages immune function; declining helper-T-cell function contributes to immunodeficiency.

Transmission routes taught by NCERT include sexual contact, infected blood, contaminated needles and mother-to-child transmission.

Cancer involves loss of normal growth control and uncontrolled proliferation.

• Benign tumour: remains relatively local.
• Malignant tumour: invades surrounding tissue and can spread.
• Metastasis: spread of malignant cells to distant sites.

Carcinogenic agents can be physical, chemical or biological. No treatment advice is provided.

Educational relationship

Substance exposure -> dependence/addiction risk -> physiological/behavioural harm -> prevention through awareness and professional support.