What Is a Pathogen?
Pathogen: A pathogen is any organism or agent capable of causing disease in a host. Pathogens include bacteria, viruses, fungi, protists, and abnormal proteins called prions.
Before the 19th century, illness was attributed to "bad air" or supernatural causes. Today, we know that infectious diseases are caused by microorganisms that invade, multiply within, and damage the body.
Types of pathogens:
- Bacteria , prokaryotic, single-celled; cause tuberculosis, cholera, strep throat
- Viruses , genetic material (DNA or RNA) surrounded by a protein coat; cannot replicate independently; cause influenza, HIV/AIDS, COVID-19
- Fungi , unicellular (yeast) or multicellular (moulds); cause athlete's foot, ringworm, thrush
- Protists , single-celled eukaryotes; Plasmodium causes malaria
- Prions , misfolded proteins; not living organisms, yet capable of causing disease (e.g., variant CJD)
Archaea, despite being microorganisms, are not known to cause disease in humans. Viruses are unique in that they lack their own metabolism , they must hijack host cell machinery to replicate.
How pathogens cause disease:
- Invasion , entry via respiratory tract, digestive system, broken skin, or mucous membranes
- Multiplication , rapid reproduction inside the host (bacteria by binary fission; viruses using host ribosomes)
- Damage , by producing toxins, destroying host cells, or triggering damaging immune responses
- Transmission , spreading to new hosts via coughing, sneezing, direct contact, or vectors (e.g., mosquitoes)
Think of pathogens as uninvited guests at a party , they enter without permission, consume resources, create chaos, and then spread the trouble to new venues (hosts).
Historical Breakthroughs: Semmelweis and Snow
Two landmark observations in the 19th century transformed our understanding of how infectious diseases spread , long before the germ theory was fully established.
Ignaz Semmelweis and Puerperal Fever
In Vienna maternity wards, Semmelweis noticed that doctors who performed autopsies and then delivered babies without washing their hands had far higher patient mortality rates from puerperal (childbed) fever. By introducing handwashing with chlorinated lime solution, he dramatically reduced deaths. His work laid the foundation for modern infection control, even though it was initially rejected by the medical establishment.
John Snow and Cholera
During a cholera outbreak in London (1854), Snow mapped the cases and traced the source to a contaminated water pump on Broad Street. By removing the pump handle, he halted the outbreak , demonstrating that cholera was waterborne, not spread through "bad air."
Both Semmelweis and Snow used careful observation and epidemiological reasoning to challenge prevailing beliefs , a reminder that scientific progress often depends on questioning accepted explanations in the light of new evidence.
Why this matters for IB Biology:
Understanding disease causation enables the development of vaccines, antibiotics, and public health strategies. The stories of Semmelweis and Snow illustrate how evidence-based thinking saves lives.