1.0 Diseases: Causes and Categories
A Disease is any condition that interferes with the normal functioning of the body. In ICSE Biology, we categorize diseases based on their duration and their ability to spread from one person to another.
1. Broad Classification
- Communicable (Infectious) Diseases: Caused by pathogens (bacteria, viruses, fungi) and spread through air, water, or physical contact.
Examples: Tuberculosis, Cholera, COVID-19. - Non-Communicable Diseases: Not spread through contact. They result from genetics, lifestyle, or nutritional deficiencies.
Examples: Diabetes, Cancer, Scurvy. - Acute vs. Chronic: Acute diseases come on suddenly and last a short time (e.g., Cold), while Chronic diseases develop slowly and persist for years (e.g., Arthritis).
1.1 The Agents of Infection
Pathogens are the biological agents that cause disease. Each type of pathogen operates differently within the host system.
| Pathogen Type | Diseases | Control Method |
|---|---|---|
| Bacteria | Typhoid, TB, Cholera | Antibiotics. |
| Viruses | AIDS, Polio, Measles | Vaccination (Antibiotics don't work). |
| Protozoa | Malaria, Amoebiasis | Vector control & Antiprotozoals. |
| Fungi | Ringworm, Athlete's foot | Antifungal creams/hygiene. |
Endemic, Epidemic, and Pandemic:
• Endemic: Constant presence in a specific area (e.g., Goitre in sub-Himalayan regions).
• Epidemic: Sudden outbreak affecting many at once (e.g., Plague).
• Pandemic: Worldwide spread (e.g., COVID-19, AIDS).
1.2 Focus: AIDS (Acquired Immuno Deficiency Syndrome)
Cause: Human Immunodeficiency Virus (HIV). It attacks Helper T-cells (WBCs), destroying the body's immune system.
Modes of Transmission:- Sexual contact with an infected person.
- Transfusion of contaminated blood.
- Use of infected needles/syringes.
- From infected mother to child through the placenta.
Note: AIDS does not spread by casual touch, hugging, or sharing food.
The Incubation Period: The time interval between the entry of a pathogen and the appearance of the first symptom. For the common cold, it's 1-3 days; for AIDS, it can be several years.
Frequently asked: "Why are viral diseases harder to treat than bacterial ones?" Bacteria have their own metabolic machinery that antibiotics can target. Viruses use the host's cell machinery, so drugs that kill viruses often harm our own cells too.
2.0 Bacterial and Protozoan Diseases
While viruses are considered borderline living, Bacteria and Protozoa are complete cellular organisms. They cause disease primarily by releasing toxins or physically destroying host tissues. In the ICSE curriculum, we focus on their life cycles and specific modes of prevention.
1. Tuberculosis and Cholera
Bacterial diseases are often related to sanitation and air quality.
- Tuberculosis (TB): Caused by Mycobacterium tuberculosis.
• Transmission: Droplet infection (coughing/sneezing).
• Prevention: BCG Vaccine (Bacillus Calmette-Guérin). - Cholera: Caused by Vibrio cholerae.
• Transmission: Contaminated food and water.
• Symptom: "Rice-water" stools leading to severe dehydration.
• Prevention: Proper sanitation and ORS (Oral Rehydration Solution).
2. Malaria: A Multi-Host Disease
Malaria is caused by the protozoan Plasmodium. It requires two hosts to complete its life cycle.
- Vector: Female Anopheles mosquito. (The male feeds on plant juices).
- The Process: The parasite first attacks the Liver cells and then multiplies in the Red Blood Cells (RBCs).
- The Chill: When RBCs burst, they release a toxin called Haemozoin, which causes the characteristic high fever and shivering.
Widal Test: A specific serological test used for the diagnosis of Typhoid fever (caused by Salmonella typhi). Typhoid is characterized by continuous high fever and "rose spots" on the abdomen.
Disease Summary Table
| Disease | Causative Agent | Type |
|---|---|---|
| Tuberculosis | Mycobacterium | Bacterium |
| Malaria | Plasmodium | Protozoan |
| Amoebiasis | Entamoeba histolytica | Protozoan |
Prophylaxis vs Therapy: Prophylaxis refers to measures taken to prevent a disease before it occurs (like taking anti-malarial drugs before traveling to an endemic area). Therapy refers to the treatment of the disease after the patient has been infected.
Frequently asked: "Why does the malaria patient feel cold even when their body temperature is very high?" This is due to the release of haemozoin and the body's internal thermostat (hypothalamus) reacting to the toxins released when RBCs are destroyed.
3.0 Non-Communicable Diseases and Lifestyle
Unlike infectious diseases, Non-Communicable Diseases (NCDs) cannot be transmitted from one person to another. They are often chronic in nature and are caused by a combination of genetic, physiological, environmental, and behavioral factors.
1. Common Lifestyle Disorders
- Diabetes Mellitus: A metabolic disorder where the Pancreas fails to produce enough Insulin, or the body cannot use it effectively. This leads to high blood glucose levels (Hyperglycemia).
- Hypertension (High Blood Pressure): A condition where the force of the blood against the artery walls is too high. If left untreated, it can lead to heart disease or stroke.
- Cancer: A group of diseases involving uncontrolled cell division. These cells can invade nearby tissues and spread to other parts of the body (Metastasis).
- Cardiovascular Diseases: Conditions affecting the heart or blood vessels, often caused by Atherosclerosis (fatty deposits in arteries).
2. Intrinsic Factors
Some diseases are not caused by lifestyle but by internal malfunctions:
- Genetic Disorders: Inherited from parents (e.g., Colour Blindness, Haemophilia).
- Allergies: An exaggerated immune response to harmless substances like pollen, dust, or certain foods. The body releases Histamine, causing inflammation.
Tumour: A mass of tissue formed by abnormal cell growth.
• Benign: Stays in one place (non-cancerous).
• Malignant: Spreads and invades other organs (cancerous).
Final Comparison
| Feature | Communicable | Non-Communicable |
|---|---|---|
| Cause | Exogenous (Germs/Pathogens). | Endogenous (Genetics/Lifestyle). |
| Transmission | Spreads from person to person. | Does not spread. |
| Prevention | Vaccination and Hygiene. | Healthy Diet and Exercise. |
Carcinogens: Agents that cause cancer. These include physical agents (UV rays), chemical agents (tobacco smoke, asbestos), and biological agents (certain viruses like HPV).
Frequently asked: "Is Goitre a communicable disease?" No. It is a Non-Communicable deficiency disease caused by a lack of Iodine in the diet, leading to the swelling of the Thyroid Gland.