Parasite: organism that benefits at the expense of host (+/- interaction). Ectoparasites: live on outer surface of host. Lice: Pediculus humanus capitis (head louse), P. humanus corporis (body louse), Phthirus pubis (pubic louse). Ticks: Ixodes (blacklegged tick - Lyme disease vector), Dermacentor, Rhipicephalus (dog tick). Mites: Sarcoptes scabiei (scabies - burrows in skin), Demodex (hair follicle mite). Fleas: Pulex irritans (human flea), Ctenocephalides felis (cat flea). Mosquitoes: temporary ectoparasites (females only, blood meal for egg development). Leeches: external bloodsuckers. Endoparasites: live inside host. Intestinal: Ascaris (roundworm), Taenia (tapeworm), hookworm (Ancylostoma), pinworm (Enterobius). Blood: Plasmodium (malaria), Wuchereria bancrofti (filariasis), Trypanosoma (sleeping sickness). Tissue: Trichinella (muscle), Cysticercus (larval tapeworm in tissues). Intracellular: Plasmodium (RBC, liver cells), Toxoplasma (various cells), Leishmania (macrophages).

Order Phthiraptera (sucking lice). Pediculus humanus capitis: head louse. Lives on scalp and hair. Feeds on blood by piercing skin. Eggs (nits) glued to hair shafts close to scalp. Highly contagious - spreads by direct head-to-head contact or sharing combs/hats. NOT a disease vector (unlike body louse). Treatment: permethrin, pyrethrin shampoos; manual removal of nits with fine-tooth comb. P. humanus corporis: body louse. Lives in clothing seams, feeds on skin. Disease vector: typhus (Rickettsia prowazekii), relapsing fever (Borrelia recurrentis), trench fever. Historical: epidemics of typhus decimated armies (more soldiers died from typhus than battle in many 19th-century wars). Phthirus pubis (pubic louse/crab louse): sexually transmitted, lives in pubic hair. All three are obligate ectoparasites - cannot survive off the host for more than 1-2 days.

Plasmodium: intracellular endoparasite. Species: P. falciparum (most dangerous, cerebral malaria, drug resistant), P. vivax (most common, relapses from liver), P. malariae (mild, can persist 30+ years), P. ovale (uncommon), P. knowlesi (zoonosis from monkeys). Life cycle: Sexual phase in Anopheles mosquito (definitive host). Asexual phase in human (intermediate host). In human: sporozoites (from mosquito bite) infect liver cells (hepatocytes) - schizogony - merozoites released - infect RBCs - erythrocytic schizogony - ring stage, trophozoite, schizont - RBC rupture releases merozoites (causes fever/chills cyclically) - some become gametocytes (picked up by mosquito). P. vivax and P. ovale: hypnozoites (dormant forms) in liver cause relapses months/years later. Symptoms: periodic fever (48h cycle for P. vivax/P. falciparum, 72h for P. malariae), chills, headache, anaemia, splenomegaly. Cerebral malaria (P. falciparum): sequestration of infected RBCs in brain capillaries, coma, death.

Taenia solium (pork tapeworm): adult in human small intestine = endoparasite. Proglottids: segments. Scolex: head with suckers and hooks for attachment. Strobiocercus: body chain of proglottids. Length: up to 7 metres. Gravid proglottids: full of eggs, shed in faeces. Life cycle: eggs ingested by pig - hatch - larvae (oncospheres) bore through intestine - travel via blood to muscles - form cysticerci (larval stage in pork). Human eats undercooked pork - cysticercus released - attaches to intestine - adult tapeworm. Cysticercosis: if human ingests eggs (faecal-oral route) - cysticerci form in human muscles and brain! Neurocysticercosis: cysts in brain = seizures, headaches, potentially fatal. Major public health problem in developing countries. Prevention: proper cooking of pork, sanitation, hygiene. Taenia saginata: beef tapeworm, no cysticercosis in humans.

Ascaris lumbricoides: most common helminth infection globally (~800 million infected). Nematode (roundworm). Adult: in small intestine of humans. Length: male 15-31 cm, female 20-35 cm (largest human intestinal nematode). Life cycle: eggs in soil (highly resistant - survive years). Ingested by humans via contaminated food/water/soil. Eggs hatch in intestine - larvae migrate through intestine wall - portal blood - liver - heart - lungs (Loeffler syndrome: pulmonary eosinophilia, cough). Larvae swallowed - reach intestine - mature to adults. Approximately 2-3 month cycle from ingestion to egg production. Heavy infection: malnutrition (competes for nutrients), intestinal obstruction (hundreds of worms), biliary obstruction. Diagnosis: stool examination (eggs visible microscopically). Treatment: albendazole, mebendazole. Prevention: sanitation, hand washing, proper disposal of human faeces.

Parasites cause harm to host: direct damage (tissue destruction), nutritional competition (absorb host nutrients), toxic secretions, immune evasion (some parasites hide from immune system). Host defences: innate immunity (physical barriers, inflammation, phagocytes), adaptive immunity (antibodies, T cells). IgE antibody: major defence against helminths. Mast cells and eosinophils: activated by IgE, attack parasites. Parasite evasion strategies: antigenic variation (Trypanosoma changes surface coat VSGs, Plasmodium changes PfEMP1 on RBCs), immunosuppression (HIV, some helminths suppress host immunity), molecular mimicry (parasite surface resembles host molecules to avoid recognition), intracellular hiding (Plasmodium inside RBCs, Leishmania inside macrophages). Parasite manipulation of host behaviour: Toxoplasma: infected rats lose fear of cats - helps parasite reach definitive host (cat). Cordyceps fungi: infect ants, cause them to climb and anchor to vegetation before killing - allows spore dispersal.

Vector: organism that transmits pathogen from one host to another. Anopheles mosquito: female transmits Plasmodium (malaria). Also vectors of filariasis (Culex), dengue (Aedes aegypti), Zika (Aedes), chikungunya (Aedes), yellow fever (Aedes), Japanese encephalitis (Culex). Body louse (Pediculus corporis): vector of typhus (Rickettsia), relapsing fever (Borrelia), trench fever. Ticks: Lyme disease (Ixodes transmits Borrelia burgdorferi), Rocky Mountain spotted fever, tick-borne encephalitis. Sandfly (Phlebotomus): vector of Leishmania (kala-azar, cutaneous leishmaniasis). Tsetse fly (Glossina): vector of Trypanosoma (sleeping sickness in Africa, Chagas disease by Triatoma bug in Americas). Flea (Xenopsylla cheopis = rat flea): vector of plague (Yersinia pestis) and murine typhus. Note: female Anopheles is a temporary ectoparasite AND the definitive host of Plasmodium. This dual role (vector + host) is unusual in parasitology.

Malaria: ~5.6 million cases/year (2021, India). Odisha, Chhattisgarh, Jharkhand, NE states most affected. P. falciparum increasingly common and drug resistant. National Vector Borne Disease Control Programme (NVBDCP). Filariasis (Wuchereria bancrofti): lymphatic filariasis. 31 million people at risk in India. Elephantiasis (lymphoedema, hydrocele). Transmitted by Culex mosquitoes. Mass Drug Administration (MDA) with DEC (diethylcarbamazine) for elimination. Kala-azar (visceral leishmaniasis): mainly Bihar, Jharkhand, West Bengal, UP. National elimination target: less than 1 case per 10,000 population per block. Soil-transmitted helminths: Ascaris, hookworm, Trichuris. High prevalence in rural India. National Deworming Day (Feb 10 and Aug 10): school-based albendazole distribution. Japanese Encephalitis: UP, Bihar, Assam. Dengue: all states, seasonal outbreaks. No vaccine available for all serotypes in India.