Microbes, disease, and public health
Microbes cause disease, but understanding them has produced some of history's greatest health gains.
Germ theory
In the 1800s, Louis Pasteur and Robert Koch showed that specific microbes cause specific diseases. Koch's postulates set criteria: find the microbe in every case, grow it in pure culture, show it causes the disease in a healthy host, and recover it again. Germ theory replaced ideas like bad air causing disease.
How diseases spread
Pathogens spread by direct contact, respiratory droplets and aerosols, contaminated food and water, and vectors such as mosquitoes that carry malaria. Cholera outbreaks in 1850s London were traced by John Snow to a single contaminated water pump, a founding moment for epidemiology.
Fungi and protists as pathogens
Fungi cause athlete's foot, thrush, and dangerous infections in people with weak immune systems. Protists cause malaria (Plasmodium, carried by mosquitoes), sleeping sickness, and amoebic dysentery. Malaria still kills hundreds of thousands of people each year, mostly young children.
Antibiotics
Alexander Fleming discovered penicillin in 1928 when mould killed bacteria on a culture plate. Antibiotics work by targeting bacterial features, such as cell walls (penicillin) or bacterial ribosomes (tetracycline), leaving human cells unharmed. They transformed medicine, making surgery and childbirth far safer.
Antibiotic resistance
Overuse and misuse of antibiotics in medicine and farming have selected for resistant bacteria such as MRSA. Resistant infections are harder to treat and more deadly. Responses include using antibiotics only when needed, finishing prescribed courses, better infection control, and developing new drugs.
Hygiene and sanitation
Handwashing, clean water, and sewage treatment have prevented more deaths than almost any medicine. Ignaz Semmelweis showed in the 1840s that doctors washing their hands sharply reduced deaths of new mothers. Food safety practices such as refrigeration and pasteurisation also prevent disease.
Epidemics and pandemics
An epidemic is a rapid increase in cases in a region; a pandemic spreads worldwide. The basic reproduction number R0 is how many people one case infects in a fully susceptible population: if it is above 1, an outbreak grows. Measures like vaccination, isolation, and masks aim to push the effective number below 1.
The microbiome
Your body hosts trillions of microbes, especially in the gut, collectively called the microbiome. They help digestion, train the immune system, and crowd out pathogens. Antibiotics can disrupt the microbiome; research links its makeup to conditions from allergies to obesity, though many links are still being studied.
Key takeaways
Germ theory and Koch's postulates link microbes to disease. Pathogens spread by contact, air, food and water, and vectors. Fungi and protists, as well as bacteria and viruses, cause disease. Antibiotics target bacterial features, but misuse breeds resistance. Sanitation, hygiene, and vaccines are powerful public health tools; R0 above 1 means an outbreak grows.
Flashcard
Specific microbes cause specific diseases
Check your understanding
A disease has an R0 of 3. Without intervention, what is expected?