Unit question: How do microscopic organisms cause disease, and how has understanding them changed the course of human history?

For most of human history, people had no idea why they got sick. In the 1860s, Louis Pasteur demonstrated that microorganisms cause disease, a discovery that changed medicine forever. Today we know that disease agents called pathogens include bacteria, viruses, fungi, and parasites. Understanding them is the key to treating and preventing infectious disease.

A pathogen is any organism or agent that causes disease. The four main types:

1. Bacteria: Single-celled microorganisms. Some cause disease by releasing toxins or destroying tissues. Treated with antibiotics. Examples: strep throat, tuberculosis, urinary tract infections.

2. Viruses: Non-cellular agents made of genetic material and a protein coat. They hijack host cells to reproduce. Antibiotics do NOT work on viruses. Examples: influenza, COVID-19, chickenpox.

3. Fungi: Multi-celled or single-celled organisms. Fungal diseases often affect the skin, lungs, or immune-compromised patients. Treated with antifungal medicines. Examples: athlete's foot, ringworm, candida.

4. Parasites: Organisms that live on or in a host organism and benefit at the host's expense. Can be microscopic (Plasmodium, which causes malaria) or larger (tapeworms). Treated with antiparasitic medicines.

Key point: Knowing the pathogen type determines the correct treatment. Taking antibiotics for a viral illness does not work and contributes to antibiotic resistance.

Transmission is how a pathogen spreads from person to person or from the environment to a person. The main routes:

- Respiratory droplets: Pathogens travel in moisture released during coughing, sneezing, or speaking. Prevention: masks, distance. - Direct contact: Pathogens transfer through touching infected skin, surfaces, or body fluids. Prevention: handwashing, gloves. - Contaminated food or water: Pathogens are ingested. Prevention: safe food handling, clean water systems. - Vector-borne: An animal (vector) such as a mosquito or tick carries and transmits the pathogen. Prevention: insect repellent, bed nets. - Airborne: Very small particles remain suspended in the air and are inhaled. Prevention: ventilation, masks.

Interrupting transmission at any step breaks the chain of infection and protects individuals and communities.

When a disease spreads, scientists use specific terms based on geographic scale:

- Endemic: A disease that occurs at a consistent, expected level within a specific geographic area. Example: seasonal flu in a region during winter. - Epidemic: A sudden increase in disease cases beyond what is normally expected in a population or region. Example: a large flu outbreak in a city. - Pandemic: An epidemic that has spread across multiple countries or continents, affecting a large portion of the world's population. Example: COVID-19 (2020), the 1918 influenza pandemic.

Epidemics and pandemics share the same biology. The difference is geographic scale. A pandemic is declared when an epidemic crosses international boundaries and spreads widely.