Key Points

Health is a state of complete physical, mental, and social well-being, and not merely the absence of disease or physical fitness.

Infectious diseases are easily transmitted from one person to another (e.g., common cold, AIDS), while non-infectious diseases are not transmissible (e.g., cancer, genetic disorders).

Typhoid is caused by Salmonella typhi and confirmed by the Widal test. Pneumonia is caused by Streptococcus pneumoniae or Haemophilus influenzae, which infect the lung alveoli.

The common cold is caused by Rhinoviruses, which infect the nose and respiratory passage but not the lungs. It spreads through droplets from coughs or sneezes.

Malaria is caused by Plasmodium, transmitted by the female Anopheles mosquito. Amoebiasis (amoebic dysentery) is caused by Entamoeba histolytica, transmitted through contaminated food and water.

Ascariasis is caused by the roundworm Ascaris. Filariasis or Elephantiasis is caused by the filarial worm Wuchereria and is transmitted by mosquito vectors.

Innate immunity is the non-specific defense present at birth. Acquired immunity is pathogen-specific, characterized by memory, and develops after exposure to an antigen.

Innate immunity includes four types of barriers: Physical (skin, mucus), Physiological (stomach acid, saliva), Cellular (leukocytes like PMNL-neutrophils), and Cytokine (interferons).

Active immunity is produced when the host body makes its own antibodies after infection or vaccination. Passive immunity is when ready-made antibodies are provided, such as IgA from mother's colostrum.

An antibody is a Y-shaped protein represented as (H_2L_2), consisting of two identical heavy (H) chains and two identical light (L) chains. Major types include IgA, IgM, IgE, and IgG.

Cell-mediated immunity (CMI) is mediated by T-lymphocytes. It is responsible for differentiating between 'self' and 'non-self' cells, leading to graft rejection during organ transplantation.

Vaccination is based on the immune system's memory. A preparation of antigenic proteins or inactivated pathogen (vaccine) is introduced to stimulate the production of antibodies and memory B and T-cells.

Allergy is an exaggerated immune response to allergens, mediated by IgE antibodies. Autoimmunity occurs when the body attacks its own cells, as seen in rheumatoid arthritis.

AIDS (Acquired Immuno Deficiency Syndrome) is caused by the Human Immunodeficiency Virus (HIV), a retrovirus. It spreads through sexual contact, contaminated blood, shared needles, and from an infected mother to her child.

HIV infects helper T-lymphocytes ((T_H) cells). The viral RNA replicates to form viral DNA using the enzyme reverse transcriptase, leading to a progressive decrease in (T_H) cells and a weakened immune system.

Cancer is characterized by uncontrolled cell growth, forming tumors. Malignant tumors invade surrounding tissues and can spread to distant sites through the blood, a property called metastasis.

Cancer-causing agents are called carcinogens and can be physical (X-rays), chemical (tobacco smoke), or biological (oncogenic viruses). Detection methods include biopsy, radiography, CT scans, and MRI.

Major groups of abused drugs are opioids (heroin), cannabinoids (marijuana, hashish), and coca alkaloids (cocaine). These drugs affect the central nervous and cardiovascular systems.

Addiction is a psychological attachment to the euphoric effects of drugs or alcohol. Dependence is a physical state where the body shows unpleasant withdrawal symptoms if the substance is discontinued.