Human Health and Disease: Infections, Cancer and Allergies — Notes

Health is a state of complete physical, mental and social well-being. Infectious diseases are caused by pathogens and spread from person to person.

Viral diseases

• Common cold — rhinoviruses; infect the nose and respiratory passages, not the lungs. Nasal congestion and discharge, sore throat, hoarseness, cough, headache and tiredness lasting 3–7 days. Spread by droplets from coughs and sneezes and by contaminated objects (pens, books, door handles).
• Chikungunya — chikungunya virus, spread by the bite of the Aedes mosquito; fever with severe, prolonged joint pain, rash.
• Dengue — dengue virus, spread by the day-biting Aedes aegypti; sudden high fever, severe headache, pain behind the eyes, joint and muscle pain (“break-bone fever”), rash, fall in platelet count; severe forms cause bleeding.

Bacterial diseases

• Typhoid — Salmonella typhi; enters the small intestine with contaminated food and water and spreads to other organs. Sustained high fever (39–40 °C), weakness, stomach pain, constipation, headache, loss of appetite; intestinal perforation and death in severe cases. Confirmed by the Widal test. Mary Mallon (“Typhoid Mary”), a cook, was a carrier who spread typhoid for years.
• Pneumonia — Streptococcus pneumoniae and Haemophilus influenzae; infects the alveoli, which fill with fluid, reducing respiration. Fever, chills, cough, headache; in severe cases lips and fingernails turn grey to bluish. Spread by droplets or by sharing glasses and utensils.
• Diphtheria — Corynebacterium diphtheriae; sore throat, fever, a tough grey membrane in the throat that may block breathing; its toxin can damage the heart and nerves. Prevented by the DPT vaccine.
• Plague — Yersinia pestis; spread by the bite of the rat flea; fever, chills, painful swollen lymph nodes (buboes) in bubonic plague; controlled by rodent control and antibiotics.

Widal test. The patient’s serum is serially diluted (1 : 20, 1 : 40, 1 : 80 …) and mixed with killed Salmonella antigens. Agglutination (clumping) shows antibodies against the bacterium. The titre is the highest dilution that still agglutinates; a high titre (for example 1 : 160) suggests active typhoid.

Amoebiasis (amoebic dysentery) — the protozoan Entamoeba histolytica in the large intestine. Constipation, abdominal pain and cramps, stools with excess mucus and blood clots. Houseflies are mechanical carriers that transfer cysts from faeces to food and water.

Malaria — Plasmodium (P. vivax, P. malariae, P. falciparum; P. falciparum causes the most serious, malignant malaria). Vector: female Anopheles mosquito. Life cycle (two hosts):

1. An infected mosquito bites and injects sporozoites (infective stage).
2. Sporozoites multiply asexually in liver cells, then attack red blood cells (RBCs), multiply there and burst them.
3. Bursting RBCs release the toxin haemozoin, which causes chill and high fever; in vivax malaria the bouts recur about every 48 hours as RBCs burst together.
4. Some parasites develop into gametocytes in RBCs.
5. A female Anopheles takes up gametocytes with blood; fertilisation and development occur in the mosquito’s stomach (gut) wall.
6. Sporozoites formed there move to the mosquito’s salivary glands, ready to infect the next person. The human is the intermediate host (asexual phase); the mosquito is the definitive host (sexual phase).

Ascariasis — the roundworm Ascaris lumbricoides in the intestine; internal bleeding, muscular pain, fever, anaemia and blockage of the intestine. Eggs passed in faeces contaminate soil, water and vegetables.

Filariasis (elephantiasis) — Wuchereria bancrofti and W. malayi; live in lymphatic vessels, mostly of the lower limbs, causing chronic inflammation and gross swelling; genital organs may be affected. Spread by the bite of the female Culex mosquito.

Ringworm — fungi of the genera Microsporum, Trichophyton and Epidermophyton; dry, scaly lesions on skin, nails and scalp with intense itching; favoured by heat and moisture (groin, between toes); spread through soil and shared towels, clothes and combs.

Prevention and control: personal hygiene (clean body, safe food and water); public hygiene (proper disposal of waste and excreta, clean water reservoirs); for air-borne diseases, avoid close contact with patients; for vector-borne diseases, remove stagnant water, use mosquito nets and screens, spray insecticides and introduce Gambusia fish that feed on mosquito larvae; vaccination and antibiotics.

Cancer is the uncontrolled division of cells. Normal cells show contact inhibition: contact with other cells stops their division. Cancer cells lose contact inhibition, divide continuously and form masses called tumours.

Benign tumours remain confined to their original location (often within a capsule), do not spread, and cause little damage. Malignant tumours are masses of proliferating neoplastic cells that grow rapidly, invade and damage surrounding tissue and compete with normal cells for nutrients. Cells sloughed from them reach distant sites through the blood, where they start new tumours: metastasis, the most feared property of malignant tumours. A tumour of about 10⁹ cells needs about 30 doublings from one cell.

Causes (carcinogens): physical — ionising radiation (X-rays, gamma rays) and non-ionising UV light damage DNA; chemical — tobacco smoke (the major cause of lung cancer) and some industrial chemicals; biological — oncogenic viruses carrying viral oncogenes. Normal cells carry proto-oncogenes (cellular oncogenes, c-onc); when activated under certain conditions they cause oncogenic transformation.

Diagnosis: biopsy and histopathology of tissue; blood and bone-marrow tests (increased cell counts in leukaemia); radiography (X-rays), computed tomography (CT) and magnetic resonance imaging (MRI); antibodies against cancer-specific antigens; molecular biology techniques to detect genes that make a person susceptible.

Treatment: surgery; radiation therapy (lethal irradiation of tumour cells, protecting normal tissue); chemotherapy (drugs, many with side effects such as hair loss and anaemia); immunotherapy with biological response modifiers such as α-interferon, which activate the immune system, since tumour cells often avoid detection.

An allergy is the exaggerated response of the immune system to certain antigens present in the environment. The substances that cause it are allergens — common ones are mites in dust, pollen and animal dander (fur and skin flakes).

The antibodies produced are of the IgE type. On a later exposure, allergens bind IgE on mast cells, which release chemicals such as histamine and serotonin.

General symptoms: sneezing, watery eyes, running nose and difficulty in breathing (as in asthma); skin rashes may occur.

Relief: drugs such as anti-histamines, adrenaline and steroids quickly reduce the symptoms. Modern, very protected lifestyles may lower immunity and increase sensitivity to allergens.