Health Immunity and Disease - External Factors Affecting Health

External factors affecting health include lifestyle choices (diet, exercise, substance use), environmental conditions (pollution, sanitation), and biological agents (pathogens).

Pathogens are microorganisms that cause infectious diseases. These include bacteria, viruses, fungi, and protists. They can enter the body through various portals of entry such as the respiratory tract, digestive system, or broken skin.

Non-communicable diseases (NCDs) are often linked to lifestyle 'risk factors'. For example, smoking is a major risk factor for lung cancer and cardiovascular disease due to chemicals like tar, nicotine, and carbon monoxide ($CO$).

Obesity is an external health factor caused by an energy imbalance where energy intake exceeds energy expenditure. It is a risk factor for Type 2 diabetes and high blood pressure.

The Body Mass Index ($BMI$) is a common tool used to estimate if a person is at a healthy weight for their height, though it does not account for muscle mass vs. fat mass.

Alcohol consumption affects the liver and brain. Ethanol ($C_2H_5OH$) is a depressant that can lead to cirrhosis (scarring of the liver) and impaired cognitive function.

Malnutrition involves a diet lacking in specific nutrients. For example, a lack of Vitamin C leads to scurvy, while a lack of Vitamin D or calcium ($Ca^{2+}$) can lead to rickets.

Correlation vs. Causation: While a correlation shows a link between an external factor (e.g., smoking) and a disease (e.g., lung cancer), causation must be proven through scientific mechanisms (e.g., carcinogens damaging $DNA$).