8.13 Pathogens and Infectious Disease

Pathogens and disease transmission

Pathogens are disease-causing organisms including bacteria, viruses, protozoa, and parasitic worms. Diseases transmit via direct contact, vectors (organisms like mosquitoes that carry pathogens between hosts), contaminated water/food, or airborne routes. Infectious diseases spread from person to person or via vectors, unlike chronic diseases caused by lifestyle or genetics.

Water-related pathogens

Poor sanitation and contaminated drinking water spread pathogens like cholera bacteria, Giardia, and Cryptosporidium, causing diarrheal diseases that kill millions, especially children in developing countries. Malaria, spread by *Anopheles* mosquitoes breeding in standing water, remains a leading vector-borne killer. Improving water treatment and sanitation infrastructure greatly reduces disease burden.

Environmental change and disease spread

Climate change and habitat disruption expand the range of disease vectors like mosquitoes into new regions as temperatures warm, increasing exposure to malaria and dengue. Deforestation and human encroachment into wildlife habitats increase contact with new pathogens (zoonotic spillover). Urbanization with inadequate sanitation concentrates populations, accelerating pathogen transmission.

Key terms

Pathogen

A disease-causing organism, such as bacteria, viruses, protozoa, or parasitic worms.

Vector

An organism (e.g., mosquito) that transmits a pathogen from one host to another.

Waterborne pathogens

Disease-causing organisms like cholera bacteria, Giardia, and Cryptosporidium spread through contaminated water.

Malaria

A vector-borne disease caused by Plasmodium parasites, transmitted by Anopheles mosquitoes breeding in standing water.

Climate change & disease spread

Warming temperatures expand mosquito and other vector habitats, increasing the geographic range of diseases like malaria and dengue.

Try an AP-style question

What is a pathogen?

1. A cancer-causing chemical
2. A greenhouse gas in the air
3. A disease-causing organism
4. A nutrient causing eutrophication