Plagues and People Notes
Chapter 1 – the Nature of Plagues:
- Symptoms of pneumonia: fever, chills, headache, muscle aches, dry cough, chest pains, shortness of
breath, vomiting, diarrhea, lungs filled with fluid and pus, confusion, disorientation, hallucinations, loss of
- In 1976 Peter Turner, a world war 2 veteran attended the Legion convention in Philadelphia and was
the first to get this disease in the hotel where he was for a convention
- Legionnaires’ disease effected 221 people and 33 died from pneumonia
- No person to person spread
- Air was the probable pathway of this disease spreading and the most popular theory was that infection
resulted from aspiration (produced by choking… secretions in the mouth get past the chocking reflex and,
instead of going into the esophagus and stomach, mistakenly enter the lungs) of bacteria (called
legionella) in aerosolized water from either cooling towers or evaporative condensers. Protective
mechanisms that normally prevent aspiration are defective in older people, smokers and people with lung
- 11,000 cases documented of Legionnaires’ disease in the US annually, and estimates of 100,000 with a
fatality rate of 15%
- Outbreaks have been traced to water heaters, whirlpool baths, respiratory therapy equipment, an
ultrasonic misters used in grocery stores.
- Toxic Shock Syndrome or TSS is a gender specific disease
- Mary Benton, graduate student and English composition TA at USLA was the first to get it
- Each case began with vomiting and high fever, followed by light headedness, fainting, soar throat, and
muscle aches. A day later victim got a rash and eyes become bloodshot red. 3-4 days later victim
suffered confusion, fatigue, weakness, thirst, rapid pulse, cool and moist skin, and rapid breathing. This
was followed by a sudden drop in blood pressure and shock if it remained long enough
- From 1979 to 1996 it effected 5296 woman generally around age 22 with a peak death rate of 4%
- TSS was not an STD
- Was linked to the use of certain kinds of tampons, especially those containing cross linked
carboxylmethyl celluslose with polyester foam, which provided a favourable environment for the toxin
producing S. aureus
- Elevated vaginal temperature and neutral pH were enhanced by the use of these super absorbent
- These tampon brands were removed from the market by the late 1800’s.
- SARS (severe acute respiratory syndrome) began in Feb 2003 when a Chinese physician who worked in
a hospital in Guangdong, southern china travelled to Hong Kong and became ill and then died. His
brother in law was in contact with him for 10 hours got it 3 days later and died 3 weeks later.
- Symptoms were fever, dry cough, sore throat, headache, and difficulty breathing
- In 4 months sars lead to 8000 cases and 800 deaths in 27 countries
- Economic cost was 100 billion$
- The germs that caused these diseases are called parasites which are unable to survive on their own and
require another living bring for their nourishment.
- Parasitism is to obtain the resources needed for growth an reproduction. The parasite benefits at the
expense of their host and often end up harming the host, causing the host to get a “disease”
- Parasites come in forms such as bacteria, fungi and protozoa (all single cell), roundworms, mosquitoes,
flies, ticks (all multi cellular) and virus.
- Some such as tapeworm, hookworm, malaria parasite and HIV live inside the body while some such as
ticks and chiggers live on the surface
- The movement of parasite from host to host is called transmission, and living organisms that are
involved in this are called vectors
- Bacteria , virus and protozoa are called macro parasites because they can’t be seen without a microscope
- Read pg 7-9 for more detail on parasites
- Plagues are “to strike a blow that wounds”. In the past the sudden increase in the number of individuals
in a population affected by a disease was called a plague. Today we call it epidemic
- Flu is contagious 24 hours before symptoms appear and has a 2-4 day incubation period
- In 1906 Mary Mallon was a carrier of typhoid germs (which causes headache, low energy, diarrhea,
fever and 10% chance of death), which she was unaffected by, but could pass it to others. She was a
cook and passed it on to many people through her cooking
- Three factors required for a parasite to spread from host to host: there must be infectious individuals,
there must be susceptible individuals, there must be a means of transmission between the two.
- Epidemiologists are disease forecasters that study the occurrence, spread and control of a disease in a
population by using statistical data to identify the causes and modes of disease transmission, to predict
the likely hood of an epidemic, to identify the risk factors, and to help plan control programs such as
quarantine and vaccination
- The number of individuals each infected person infects at the beginning of an epidemic is given by R₀;
this is the basic reproductive ratio of the multiplier of the disease
- Multiplier helps predict how fast disease will spread through population
- Increase in population size or rate of transmission increases R₀, where as increase in parasite molarity
or decrease in transmission will reduce the spread of disease in a population
- Change that increases the value of R₀ tends to increase the portion of hosts infected (prevalence) as
well as the burden (incidence) of a disease
- If value of R₀ is greater than one, the “seeds” of the infection (transmission stages) will lead to an ever-
expanding spread of the disease. If R₀ is less than one, each infection produces fewer than one
transmission stage and the parasite cannot establish itself.
- Measles are caused by virus
- Transmitted through the air as a fine mist released through coughing, sneezing and talking
- Virus first reaches upper respiratory track, and then lower and multiplies for 2-4 days and then spreads
to the lymph nodes. Victim shows no sign of disease yet
- Virus invades white blood cells and are carried to all parts of body using the bloodstream as a
- After incubation period of 8-12 days, victim gets fever, weekness, loss of appitite, coughing, runny
nose, watery eyes. Virus replication is now in higher gear.
- A telltale rash appears first on ears and forhead, and then on face, neck, trunk and feet.
- Once begun, measles cannot be stopped from spreading to the body
- Death rate of 30% in undeveloped countries. One of the 10 most frequent causes of death in the world
- Type 1 epidemic: population is large and pattern shows a regular series of outbreaks/peaks, but disease
never completely disappears. This is because the number of susceptible individuals is large enough for
the chain of transmission of infection to remain unbroken, so R₀ is greater than one.
- Type 2 epidemic: peaks of infection are discontinuous, but there is a regular pattern of occurance of
cases. There is temporary absences of the disease and R₀ is less than one. This pattern occurs because
there are not enough susceptible individuals to maintain the chain of virus transmission
- Type 3 epidemic: occurs in communities with less than 10000 people. The pattern of an increased
number of cases occurs at irregular intervals, and there are long periods when there are no diseases. R₀
is much smaller than one.
- Pandemic: when an infectious disease becomes worldwide or widespread and R₀ is a large number. Ex:
SARS, influenza, bubonic plague, cholera, AIDS
- Look at figure 1.5 on page 16
- Herd immunity: For disease elimination, not EVERYONE in the population must be immunized, but
immunization is necessary to reduce the number of at risk individuals bellow a critical point. - The longer the host lives, the greater the opportunity for the parasite to grow, reproduce, and disperse
its infective stages to new hosts
- Most successful parasites are ones that cost the least harm to the host, and that overtime there is a
tendency for powerful parasites to become less powerful
- In 1859 a ship of rabbits came from England to Australia, and since rabbits did not have preditors in
Australia they multiplied rapidly destroying plants and native animals
- The Austins ended up killing over 20,000 rabbits by 1865, and then the viral disease myxomatosis was
introduced from South America in 1950 to act as a biological control agent, and killed 99% of the rabbits.
- Several years later the virus only killed 90% of the rabbits and the viruses where less powerful and
were much better being transmitted by mosquitoes, the vector for the virus.
- The rabbits lived longer and the virus evolved towards gentle coexistence with the rabbit host.
- Although the power of parasites tends to decline over evolutionary time, it never becomes entirely
powerless; in the process, the parasite population becomes more efficient in regulating the size of the at
risk host population
- The number of parasite offspring lost is the number of offspring that pass on their genes to succeeding
generations that determines evolutionary success. Natural selection favours those characteristics that
increase the passing on of a specific set of genes.
- If host resistance is low, the disease is more pathogenic. How pathogenic the disease is is determined
by virulence (how powerful) and host resistance
- Sneezing, coughing and diarea assist in parasite transmission
Chapter 2 – Plagues, the Price of Being Sedentary:
- Africa was the cradle of humanity
- Our oldest ancestors lived in Africa 4.2 million years ago and are named Australopithecus anamnesis.
- 2.5-1.8 million years ago, they found a species “homo habilis” (handy man). This name is given because
altered stones and animal remains where found with the fossils.
- H.habilis has zoonotic infections, which means that parasites were acquired from the wild animals that
they killed and hunted.
- H.habilis might have suffered viral diseases such as mosquito transmitted yellow fever, hepatitis,
- They probably did not have typhus, mumps, measles, influenza, tuberculosis, cholera, chicken pox,
diphtheria or gonorrhea
- 1.6 million years ago, H.habilis was replaced by Homo erectus (erect man), who was closer to modern
humans in body size and had a brain bigger than .Habilis, but with half the capacity of modern humans.
- Read page 30-31 ; more humans, more disease
- Agricultural revolution: humans going from hunter-gatherers to farmers.
- Pattern an impact on desease depends on: population density; quality of food, water and shelter;
frequency of contact amongst individuals; human contact with animals; climate.
- People infected with intestinal parasites can transmit the disease to others through their feces, and
when water supply becomes contaminated either th