Infectious disease
-#1 killer worldwide
-#3 in United States: only behind heart disease and cancers
Infectious Disease is caused by microbes
-bacteria
-viruses
-protozoa and fungi
Major killers of humanity evolved from animal diseases
How Evolution Works
-evolution is descent with modification
-accumulation of mistakes over time
-those mistakes give some advantage over others allowing organisms to survive and reproduce
-reproduction (transfer of genes) is what passes the advantage on to the next generation
Evolution selects for those who are best at producing babies (offspring) and helping them spread to a suitable place to live and reproduce — spread genes
Evolution of microbes
-the germ that spreads better leaves more babies and ends up being selected for
From the perspective of a microbe
-Examples of how successful germs evolved to spread themselves
1. “Symptoms” of Disease
-clever ways that microbes modify our bodies to spread themselves
-coughing, sneezing, runny nose, mucus (flu, colds)
-rashes and itching, open sores (syphilis, herpes, smallpox)
-massive diarrhea – cholera
-5 million children under 5 die every year from diarrhea (poor
sanitation)
-sex – they spread themselves as we are attempting to spread ourselves
(STD’s)
2. wait to be eaten
-Salmonella, trichinosis
3. hitchhike in the saliva of an insect
-mosquitoes (malaria), fleas (bubonic plague), tsetse fly (sleeping
sickness)
4. Mother to baby – Birth
-syphilis, herpes, AIDS
5. Rabies
Human evolution
-over history some people are just more genetically resistant to certain microbes, therefore they survive and pass on their genes to their babies
Epidemics or “Crowd Diseases” – measles, smallpox, influenza
-spread quickly
-people either die or recover
-those who recover become resistant (once you get measles you don’t get it again)
-Vaccines
-survive only in humans, not animals or soil
-disease dies out until new crop of babies reaches susceptible age and an outsider arrives to start new epidemic
-large population needed to sustain epidemic ( > 500,000)
-if population is big enough (like US) then disease can move among groups
-epidemics could not be sustained in hunter-gatherers
-rise of agriculture —- rise of cities (population) — rise of disease
-travel spreads disease
Epidemic diseases evolved from animal disease
-germ makes jump from animal to human
Human disease and animal with related pathogen
Measles from cattle
Tuberculosis from cattle
Small pox from cattle
Flu from pigs and ducks
AIDS from monkeys
1918 Flu
-killed 21 million
-bird flu
-was able to spread from human to human
The disease represents an evolution in progress.
We fight back.
–Immune systems
-Individual genetic differences allow some people to be more resistant
-these people then pass on their advantage
–Our genes determine what we can fight
-As germs are modified with descent to gain advantages, we must also be modified
We are modified through natural selection (annihilation of civilizations) or technology (antibiotics/medicines)
Germs continue to modify so they can use us to spread their babies
-flu, AIDS, antibiotic-resistant bacteria
