(a) What do you mean by infectious and communicable diseases? Mention the causative agent and means of spread of filaria. Also…

(a) Infectious and communicable diseases; filariasis

An infectious disease results from the invasion and multiplication of a pathogenic agent (virus, bacterium, fungus, protozoan or helminth) in the host, whatever the route of acquisition; tetanus, acquired from spores in soil, is infectious but not passed from person to person. A communicable disease is an infectious disease whose agent or its toxins can be transmitted, directly or indirectly, from an infected person, animal or reservoir to a susceptible host, as in tuberculosis, malaria and filariasis.

Causative agent: lymphatic filariasis is caused by thread-like nematodes living in the lymphatic system: Wuchereria bancrofti (about 90% of cases worldwide and most cases in India), Brugia malayi (pockets in Kerala and a few other states) and Brugia timori (Indonesia). Adults live in lymph vessels and nodes, and the females release sheathed microfilariae into the blood; in India these show nocturnal periodicity, appearing in peripheral blood mainly between about 10 pm and 2 am.

Means of spread: the disease is spread only by mosquitoes. In India W. bancrofti is transmitted mainly by Culex quinquefasciatus, which breeds in polluted water, drains and cesspits, and B. malayi by Mansonia species associated with aquatic plants such as Pistia; Anopheles and Aedes are vectors elsewhere. Microfilariae taken up in a blood meal develop in the mosquito's thoracic muscles to infective third-stage (L3) larvae in about 10–14 days; during a later bite the L3 larvae escape onto the skin and enter through the puncture, reaching the lymphatics, where they mature in months. There is no multiplication inside the mosquito, so repeated bites over years are needed.

Symptoms:

• Asymptomatic microfilaraemia: many carriers show no signs but have hidden lymphatic and kidney damage and are the reservoir of infection.
• Acute stage: recurrent filarial fever with painful lymphangitis and lymphadenitis (acute adenolymphangitis), often worsened by secondary bacterial infection, and funiculitis, epididymo-orchitis in men.
• Chronic stage: lymphoedema progressing to elephantiasis of legs, arms, scrotum or breast, hydrocele (the commonest chronic sign in bancroftian filariasis), and chyluria.
• Tropical pulmonary eosinophilia: nocturnal cough, wheeze and very high eosinophil counts from an immune reaction to microfilariae in the lungs.

Diagnosis: a thick night blood smear stained with Giemsa, or membrane filtration of venous blood, shows sheathed microfilariae; circulating filarial antigen of W. bancrofti is detected at any time of day by rapid card tests (Filariasis Test Strip); antibody tests help for Brugia; and ultrasound of the scrotum or breast can show living adult worms moving (the "filarial dance sign").

Treatment: diethylcarbamazine (DEC, 6 mg/kg daily for 12 days) kills microfilariae and some adults; albendazole and ivermectin are used in combination; doxycycline for 4–6 weeks kills adult worms by eliminating their Wolbachia endosymbionts. Chronic disease needs morbidity management: limb hygiene, skin care, exercise and elevation to prevent acute attacks, and surgery for hydrocele.

Prevention and control:

• Mass drug administration (MDA): annual single-dose treatment of the whole at-risk population with DEC plus albendazole, or the triple-drug regimen of ivermectin, DEC and albendazole (IDA), which India began in 2018, to interrupt transmission.
• Vector control: source reduction and drainage, anti-larval measures such as temephos and biological control with larvivorous fish (Gambusia, Poecilia), polystyrene beads in pits and wells, removal of Pistia for Mansonia, and personal protection with bed nets and repellents.
• Programme: the National Center for Vector Borne Diseases Control runs the elimination programme, aiming to eliminate lymphatic filariasis as a public health problem in India by 2027, ahead of the global 2030 target, with transmission assessment surveys to decide when MDA can stop and morbidity management and disability prevention for existing patients.

(b) Carbon cycle

The carbon cycle is the movement of carbon between the atmosphere, living organisms, soils, oceans and rocks. Its main reservoirs, in gigatonnes of carbon (GtC), are roughly: the atmosphere about 870–880 GtC (CO₂ over 420 ppm in 2025); vegetation 450–650 GtC; soils about 1,500–2,400 GtC; the oceans about 38,000 GtC, mostly as dissolved bicarbonate in deep water; fossil fuel deposits over 1,000 GtC; and sedimentary rocks (limestone and kerogen) tens of millions of GtC.

• Photosynthesis: land plants take up about 120 GtC a year and marine phytoplankton about 50 GtC a year, converting CO₂ into carbohydrates.
• Respiration and decomposition: autotrophic respiration, respiration by animals, and decomposition of dead organic matter by bacteria and fungi return nearly the same amount to the atmosphere; some carbon is released as methane from wetlands and ruminants.
• Ocean exchange: CO₂ dissolves in surface water (the solubility pump), forming carbonic acid, bicarbonate and carbonate. The biological pump carries organic matter and carbonate shells of plankton into deep water; a small fraction is buried in sediments.
• Sedimentation and lithification: over millions of years carbonates form limestone, and buried organic matter forms coal, oil and gas; weathering of rocks and volcanic outgassing return carbon slowly (the long-term geological cycle).
• Human perturbation: fossil-fuel burning and cement production (about 10 GtC a year) and land-use change (about 1 GtC a year) add carbon faster than natural sinks absorb it. Roughly a quarter is taken up by land, a quarter by the ocean, and nearly half remains in the atmosphere, strengthening the greenhouse effect and causing ocean acidification.

(c) Sexual conflict and selfishness in animal behaviour

Selfishness: natural selection acts on genes, and behaviour that increases an individual's own genetic success is favoured even when it harms others (Dawkins's "selfish gene"). Examples are Hamilton's selfish herd, in which each animal moves towards the centre of a group to put others between itself and a predator, and cuckoos laying eggs in other birds' nests. Apparent altruism is explained by kin selection (Hamilton's rule, rB > C), as in sterile worker honeybees helping their sisters, and by reciprocal altruism, as in blood-sharing vampire bats. Selfish genetic elements act even within the genome: meiotic drivers such as the *t*-haplotype in mice and Segregation Distorter in Drosophila are transmitted to more than half of the offspring at a cost to the carrier.

Sexual conflict: because the reproductive interests of males and females differ (anisogamy; males usually gain from many matings, females from fewer, well-chosen ones and from survival), a trait that benefits one sex can harm the other. In intralocus conflict the same gene has opposite optima in the two sexes; in interlocus conflict different genes in males and females are locked in an arms race of adaptation and counter-adaptation (chase-away selection).

• Drosophila melanogaster: seminal-fluid proteins such as sex peptide raise the female's egg-laying and reduce her remating, helping the male's sperm, but shorten female lifespan.
• Water striders (Gerris, Aquarius): males grasp females forcibly, and females have evolved abdominal spines and shape changes that resist costly extra matings.
• Bedbugs (Cimex lectularius): males pierce the female's body wall (traumatic insemination); females have evolved a spermalege that reduces wound damage and infection.
• Infanticide: a new male Hanuman langur (Semnopithecus entellus, studied by Sarah Hrdy at Abu, Rajasthan) or lion kills unweaned infants sired by the previous male, bringing the mother back into oestrus sooner; females counter by forming alliances, pseudo-oestrus and mating with several males to confuse paternity.
• Mate guarding and sperm competition: males guard females after mating (e.g., dragonflies in tandem) or remove rival sperm with specialised genitalia (damselflies), while females may bias fertilisation through cryptic female choice.
• Parental care conflict: each parent benefits if the other does more; in penduline tits either parent may desert the clutch, leaving it to the other.

Such conflicts explain the rapid evolution of reproductive proteins and genitalia and can contribute to speciation.