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7.1 Common Diseases in Humans

Human beings are susceptible to a wide range of diseases caused by various pathogenic organisms including bacteria, viruses, protozoans, fungi, and helminths. These pathogens invade the body through different routes such as contaminated food, water, air, or vectors like mosquitoes. Once inside the body, they multiply and interfere with normal physiological functions, leading to disease symptoms. Bacterial diseases include typhoid caused by Salmonella typhi, which enters through contaminated food and water, leading to symptoms like sustained high fever (39° to 40°C), weakness, stomach pain, constipation, headache, and loss of appetite. Diagnosis can be confirmed by the Widal test. Pneumonia, caused by bacteria such as Streptococcus pneumoniae and Haemophilus influenzae, infects the alveoli of lungs causing fluid accumulation and respiratory distress, with symptoms including fever, chills, cough, and headache. Viral diseases include the common cold caused by rhinoviruses, which infect the nose and respiratory passages but not the lungs. Transmission occurs through droplets from coughs or sneezes or contact with contaminated objects. Symptoms include nasal congestion, sore throat, cough, headache, and tiredness lasting 3-7 days. Protozoan diseases include malaria caused by Plasmodium species (P. vivax, P. malariae, P. falciparum). The life cycle involves two hosts: humans and female Anopheles mosquitoes. Sporozoites enter humans via mosquito bites, multiply in liver cells, then infect red blood cells causing their rupture, leading to chills and fever recurring every 3-4 days. The parasite completes its cycle in mosquitoes, which act as vectors. Amoebiasis is caused by Entamoeba histolytica, a protozoan parasite in the large intestine, leading to symptoms like constipation, abdominal pain, cramps, and stools with mucus and blood. Transmission occurs through contaminated food and water via mechanical carriers like houseflies. Helminthic infections include ascariasis caused by Ascaris roundworms, leading to symptoms such as internal bleeding, muscular pain, fever, anemia, and intestinal blockage. Eggs are excreted in feces contaminating soil and water. Filarial worms (Wuchereria bancrofti and W. malayi) cause elephantiasis or filariasis, characterized by chronic inflammation and swelling of lymphatic vessels, especially in lower limbs. Transmission is through mosquito bites. Fungal infections such as ringworm are caused by Microsporum, Trichophyton, and Epidermophyton species, leading to dry, scaly lesions on skin, nails, and scalp accompanied by itching. These fungi thrive in moist, warm areas like skin folds and are transmitted through contact with infected persons or contaminated objects. Prevention and control of infectious diseases rely heavily on personal hygiene (clean body, safe food and water), public hygiene (proper waste disposal, disinfection of water bodies), vector control (eliminating mosquito breeding sites, use of mosquito nets, insecticides), and immunisation. Vaccines have been instrumental in eradicating diseases like smallpox and controlling others like polio and tetanus. Antibiotics and other drugs help treat bacterial infections effectively.

• Pathogens include bacteria, viruses, protozoans, fungi, and helminths causing human diseases.
• Typhoid fever is caused by Salmonella typhi; diagnosed by Widal test.
• Malaria is caused by Plasmodium species transmitted by female Anopheles mosquitoes.
• Filarial worms cause elephantiasis transmitted by mosquitoes.
• Ringworm is a common fungal infection causing scaly lesions and itching.
• Prevention includes hygiene, vector control, vaccination, and use of antibiotics.
• 📌 Pathogen: Disease-causing organism.
• 📌 Vector: Organism transmitting a pathogen.
• 📌 Sporozoite: Infectious form of Plasmodium entering humans.

7.2 Immunity

Immunity is the body's ability to defend itself against disease-causing agents or pathogens. Despite daily exposure to numerous microbes, only a few cause disease due to the immune system's protective mechanisms. Immunity is broadly classified into innate (non-specific) and acquired (specific) immunity. Innate immunity is present from birth and provides the first line of defense through various barriers: (i) Physical barriers such as intact skin and mucus lining in respiratory, gastrointestinal, and urogenital tracts trap and prevent entry of microbes. (ii) Physiological barriers include acidic gastric juice, saliva, and tears that inhibit microbial growth. (iii) Cellular barriers involve phagocytic white blood cells like neutrophils, monocytes, macrophages, and natural killer cells that engulf and destroy pathogens. (iv) Cytokine barriers involve interferons secreted by virus-infected cells that protect neighboring cells from viral infection. Acquired immunity develops after exposure to specific pathogens and is characterized by memory, allowing a faster and stronger response upon subsequent exposures. It involves two types of lymphocytes: - B-lymphocytes produce antibodies (immunoglobulins) that neutralize pathogens in the blood (humoral immunity). - T-lymphocytes mediate cell-mediated immunity (CMI), attacking infected cells directly and playing a role in graft rejection. Antibodies are Y-shaped proteins composed of four peptide chains (two heavy and two light chains) and exist in various classes such as IgA, IgM, IgE, and IgG. The immune response can be active, where the body produces antibodies after exposure to antigens, or passive, where preformed antibodies are received (e.g., through mother's milk or antiserum injections). Vaccination exploits immune memory by introducing weakened or inactivated pathogens or their antigenic components to stimulate antibody production without causing disease. Recombinant DNA technology has enabled production of safer vaccines like the hepatitis B vaccine. Allergies are exaggerated immune responses to harmless environmental antigens (allergens) mediated by IgE antibodies, causing symptoms like sneezing and wheezing due to histamine release. Autoimmune diseases occur when the immune system mistakenly attacks self-cells, as in rheumatoid arthritis. The immune system comprises primary lymphoid organs (bone marrow and thymus) where lymphocytes mature, and secondary lymphoid organs (spleen, lymph nodes, tonsils, Peyer's patches, appendix) where immune responses are initiated. Mucosa-associated lymphoid tissue (MALT) lines major tracts and constitutes about 50% of lymphoid tissue.

• Immunity protects the body from pathogens and is of two types: innate and acquired.
• Innate immunity includes physical, physiological, cellular, and cytokine barriers.
• Acquired immunity is specific, has memory, and involves B and T lymphocytes.
• Antibodies are proteins produced by B cells to neutralize pathogens.
• Vaccination induces active immunity by stimulating memory cells.
• Allergies are hypersensitive immune responses; autoimmune diseases involve self-attack.
• 📌 Innate immunity: Non-specific defense present at birth.
• 📌 Acquired immunity: Specific immunity developed after exposure.
• 📌 Antibody: Protein produced by B cells to neutralize antigens.