Unit 7: Human Health and Diseases

1. Common Diseases in Humans

Health is often defined simply as absence of disease or physical fitness, but it is properly a state of complete physical, mental and social well-being. Diseases that can easily be transmitted from one person to another are called infectious diseases. Pathogens are disease-causing organisms.

Typhoid

Typhoid is a pathogenic bacterial disease affecting humans.

• Pathogen: Salmonella typhi
• Transmission: Enters the small intestine through food and water contaminated with feces, and migrates to other organs through blood.
• Symptoms: Sustained high fever (39°C to 40°C), weakness, stomach pain, constipation, headache and loss of appetite. In severe cases, intestinal perforation and death may occur.
• Exam Note: The classic diagnostic test for typhoid is the Widal test.

Pneumonia

Pneumonia is a bacterial respiratory disease.

• Pathogen: Streptococcus pneumoniae and Haemophilus influenzae
• Infection Site: Infects the alveoli (air sacs) of the lungs. The alveoli get filled with fluid leading to severe problems in respiration.
• Transmission: Inhaled droplets/aerosols released by an infected person or sharing glasses and utensils with an infected person.
• Symptoms: Fever, chills, cough and headache. In severe cases, the lips and fingernails may turn gray to bluish in color.

Common Cold

The common cold is an infectious viral ailment.

• Pathogen: Rhino viruses
• Infection Site: Infects the nose and respiratory passage, but not the lungs.
• Transmission: Droplets resulting from cough or sneeze of an infected person are either inhaled directly or transmitted through contaminated objects such as pens, books, cups, doorknobs, and computer keyboards.
• Symptoms: Nasal congestion and discharge, sore throat, hoarseness, cough, headache, tiredness, etc., usually lasting for 3-5 days.

Malaria

Malaria is a protozoan disease that has historically troubled humanity.

• Pathogen: Different species of Plasmodium, namely Plasmodium vivax, P. malariae, and P. falciparum. Falciparum malaria is the most serious one and can be fatal.
• Vector: Female Anopheles mosquito transmits the infectious form.
• Life Cycle/Step-by-Step:
  • Plasmodium enters the human body as sporozoites through the bite of an infected female Anopheles mosquito.
  • The parasites initially multiply within the liver cells and then attack red blood cells (RBCs) resulting in their rupture.
  • The rupture of RBCs is associated with release of a toxic substance, hemozoin, which is responsible for the chill and high fever recurring every three to four days.
  • The parasite requires two hosts—human and mosquito—to complete its life cycle. The gametocytes develop in human RBCs.

Amoebiasis (Amoebic Dysentery)

Amoebiasis is a parasitic infection of the gut.

• Pathogen: Entamoeba histolytica
• Symptoms: Constipation, abdominal pain and cramps, stools with excess mucus and blood clots.
• Transmission: Houseflies act as mechanical carriers and serve to transmit the parasite from feces of infected persons to food and food products, contaminating them. Drinking water and food contaminated by the feces are the main sources of infection.

Ascariasis

Ascariasis is caused by an intestinal parasite.

• Pathogen: Ascaris lumbricoides (Common roundworm)
• Symptoms: Internal bleeding, muscular pain, fever, anemia and blockage of the intestinal passage.
• Transmission: The eggs of the parasite are excreted along with the feces of infected persons which contaminate soil, water, plants, etc. A healthy person acquires this infection through contaminated water, vegetables, fruits, etc.

Elephantiasis (Filariasis)

Elephantiasis is a chronic inflammatory condition.

• Pathogen: Wuchereria bancrofti and Wuchereria malayi (Filarial worms)
• Infection Site: The worms live in the lymphatic vessels of the lower limbs for many years.
• Symptoms: Chronic inflammation of the organs in which they live for years, usually the lymphatic vessels of the lower limbs resulting in gross deformities; genital organs may also be affected.
• Transmission: Transmitted to a healthy person through the bite of female mosquito vectors.

Ringworm

Ringworm is a common fungal infection.

• Pathogen: Fungi belonging to the genera Microsporum, Trichophyton, and Epidermophyton.
• Symptoms: Appearance of dry, scaly lesions on various parts of the body such as skin, nails and scalp. These lesions are accompanied by intense itching.
• Transmission: Heat and moisture help these fungi to grow, which makes them thrive in skin folds such as those in the groin or between the toes. They are acquired from soil or by using towels, clothes or even the comb of an infected individual.

2. Immunity

Overall ability of the host to fight the disease-causing organisms, conferred by the immune system, is called immunity. Immunity is broadly classified into two categories: Innate immunity and Acquired immunity.

Innate Immunity

Innate immunity is non-specific type of defense that is present at the time of birth. It is accomplished by providing different types of barriers to the entry of the foreign agents into our body.

• Physical Barriers: Skin on our body is the main barrier which prevents entry of micro-organisms. Mucus coating of the epithelium lining the respiratory, gastrointestinal and urogenital tracts also help in trapping microbes entering our body.
• Physiological Barriers: Acid in the stomach, saliva in the mouth, tears from eyes—all prevent microbial growth.
• Cellular Barriers: Certain types of leukocytes (WBC) in our blood like polymorphonuclear leukocytes (PMNL-neutrophils), monocytes, and natural killer (type of lymphocytes) and macrophages in tissues can phagocytose and destroy microbes.
• Cytokine Barriers: Virus-infected cells secrete proteins called interferons which protect non-infected cells from further viral infection.

Acquired Immunity

Acquired immunity is pathogen-specific. It is characterized by memory. When our body encounters a pathogen for the first time it produces a response called the primary response which is of low intensity. Subsequent encounter with the same pathogen elicits a highly intensified secondary or anamnestic response due to our body remembering the first encounter.

Cells of Acquired Immunity

• B-lymphocytes: Produce an army of proteins called antibodies in response to pathogens into our blood to fight them. Each antibody molecule has four peptide chains, two small and two heavy (denoted as H2L2). Examples of antibodies produced are IgA, IgM, IgE, and IgG. This is called humoral immune response.
• T-lymphocytes: Do not secrete antibodies, but help B cells produce them. They are responsible for cell-mediated immunity (CMI), which is responsible for the graft rejection in organ transplants.

Active and Passive Immunity

• Active Immunity: When a host is exposed to antigens, which may be living or dead microbes or other proteins, antibodies are produced in the host body. It is slow and takes time to give its full effective response. Injecting microbes deliberately during immunization or natural infection induces active immunity.
• Passive Immunity: When ready-made antibodies are directly given to protect the body against foreign agents, it is called passive immunity. Example: Colostrum secreted by the mother during the initial days of lactation has abundant antibodies (IgA) to protect the infant. Fetus also receives some antibodies from mother through the placenta during pregnancy.

3. AIDS

AIDS stands for Acquired Immuno Deficiency Syndrome. It means deficiency of the immune system, acquired during the lifetime of an individual indicating that it is not a congenital disease.

• Pathogen: HIV (Human Immunodeficiency Virus), a member of a group of viruses called retrovirus, which have an envelope enclosing the RNA genome.
• Transmission:
  • Sexual contact with an infected person.
  • Transfusion of contaminated blood and blood products.
  • Sharing infected needles as in the case of intravenous drug abusers.
  • From infected mother to her child through placenta.
• High-Risk Groups: Individuals who have multiple sexual partners, drug addicts taking drugs intravenously, individuals who require repeated blood transfusions, and children born to an HIV infected mother. Note that AIDS/HIV does not spread by mere touch or physical contact; it spreads only through body fluids.
• Mechanism of Infection: After getting into the body of the person, the virus enters macrophages where RNA genome of the virus replicates to form viral DNA with the help of the enzyme reverse transcriptase. This viral DNA gets incorporated into host cell's DNA and directs the infected cells to produce virus particles. The macrophages continue to produce virus and in this way act like a HIV factory. Simultaneously, HIV enters helper T-lymphocytes (TH), replicates and produces progeny viruses. Thus, the number of helper T-lymphocytes decreases progressively in the body of the infected person.
• Symptoms and Diagnosis: Due to decrease in helper T-lymphocytes, the person starts suffering from infections that could have been otherwise overcome such as bacteria like Mycobacterium, viruses, fungi and even parasites like Toxoplasma. The patient becomes so immuno-deficient that s/he is unable to protect against any infection. The widely used diagnostic test for AIDS is ELISA (Enzyme Linked Immunosorbent Assay). Treatment with anti-retroviral drugs is only partially effective and can only prolong the life of the patient, but not prevent death.

4. Cancer

Cancer is one of the most dreaded diseases of human beings and is a major cause of death all over the globe. In our body, cell growth and differentiation is highly controlled and regulated. In cancer, there is a breakdown of these regulatory mechanisms.

• Tumor Types:
  • Benign tumors: Normally remain confined to their original location, do not spread to other parts of the body and cause little damage.
  • Malignant tumors: A mass of proliferating cells called neoplastic cells which grow very rapidly, invading and damaging the surrounding normal tissues. As these cells actively divide and grow they also starve normal cells by competing for vital nutrients. Cells sloughed from such tumors reach distant sites through blood, and wherever they get lodged in the body they start a new tumor. This property is called metastasis, which is the most feared property of malignant tumors.
• Causes of Cancer (Carcinogens): Transformation of normal cells into cancerous neoplastic cells may be induced by physical, chemical or biological agents called carcinogens.
  • Ionizing radiations: X-rays and gamma rays.
  • Non-ionizing radiations: UV rays (causes DNA damage).
  • Chemical carcinogens: Tobacco smoke (a major cause of lung cancer).
  • Biological agents: Cancer-causing viruses called oncogenic viruses have genes called viral oncogenes. Several genes called cellular oncogenes (c-onc) or proto-oncogenes have been identified in normal cells which, when activated under certain conditions, could lead to oncogenic transformation of the cells.
• Cancer Detection and Diagnosis:
  • Biopsy and histopathological studies of the tissue and blood/bone marrow tests for increased cell counts (like in leukemias).
  • Radiography (use of X-rays), CT (computed tomography) scan, and MRI (magnetic resonance imaging) to detect cancers of internal organs.
  • Antibodies against cancer-specific antigens are also used for detection of certain cancers.
• Treatment of Cancer:
  • Surgery: Tumors are surgically removed.
  • Radiotherapy: Tumor cells are irradiated lethally, taking proper care of the normal tissues surrounding the tumor.
  • Chemotherapy: Drugs are used to kill cancerous cells. Some drugs are specific for particular tumors. Side effects include hair loss, anemia, etc.
  • Immunotherapy: Patients are given biological response modifiers such as alpha-interferon which activate their destruction of tumor cells and help destroy the tumor.

5. Drugs and Alcohol Abuse

The drugs, which are commonly abused, are opioids, cannabinoids, and coca alkaloids. Majority of these are obtained from flowering plants. Some are obtained from fungi.

Major Categories of Abused Substances

Tobacco

Tobacco contains a large number of chemical substances including nicotine, an alkaloid. Nicotine stimulates adrenal gland to release adrenaline and nor-adrenaline into blood circulation, both of which raise blood pressure and increase heart rate. Smoking is associated with increased incidence of cancers of lung, bladder, throat, bronchitis, emphysema, coronary heart disease, gastric ulcer, etc. Tobacco chewing is associated with increased risk of cancer of the oral cavity.

Adolescence and Drug/Alcohol Abuse

Adolescence means a period during which a child becomes mature through the process of development. It is a transitional phase of growth and development between childhood and adulthood (generally between 12-18 years of age). Curiosity, need for adventure and excitement, and experimentation are common causes, which motivate youths towards drug and alcohol use.

Effects of Drug/Alcohol Abuse

• Reckless behavior, vandalism and violence.
• Excessive doses of drugs may lead to coma and death due to respiratory failure, heart failure or cerebral hemorrhage.
• A combination of drugs and alcohol intake leads to overdose-related deaths.
• Chronic use of drugs and alcohol damages nervous system and liver (cirrhosis).
• The chronic users share common needles while taking drugs intravenously; this spreads infectious diseases like AIDS and Hepatitis B (which are infections transmitted through blood).

Prevention and Control

• Avoid undue peer pressure.
• Education and counseling to face problems, stresses, and to channelize child's energy into sports, reading, music, yoga and other extra-curricular pursuits.
• Seeking professional and medical help whenever required (from psychologists, psychiatrists, and de-addiction programs).
• Looking for danger signs by parents and peers to bridge gaps in time.