Epidemiology of Pandemics

By pjain      Published March 27, 2020, 8:55 p.m. in blog Health   

Part of Germ Theory Series

Germ spreading Routes

Food Transmission

Chicken skin from a retail package contaminated with bacteria; some of the bacteria are Salmonella. Cooking the chicken properly will kill the bacteria and make the meat safe to eat.

Just like humans, microbes can get nutrients for their growth and can thrive on the same foods as humans. In specifically, if microbes multiply in animals and humans, they can and will RAPIDLY thrive on dairy products and meat products. Other microbes can survive but at lower rates than on meat products.

This is one reason why elevated temperatures are needed to properly cook meat so microbes are destroyed.

  1. Modern food supply chains are very complex, from ‘plough to plate’. The "frozen chain" with accountability and batch numbers has been developed to allow great care to be taken at every stage of food production to ensure that harmful microbes are not allowed to survive and multiply.

  2. Microbes can be spread from one food to another during the preparation process, for example by unclean hands, or dirty kitchen utensils, and cause illness when those foods are eaten. This is known as cross-contamination.

  3. Food Preparation. Germs spread by food, typically can cause the unpleasant symptoms of food poisoning such as sickness and diarrhoea, when the contaminated food is eaten.


Some diseases are caused by drinking water that is contaminated by human or animal faeces, which may contain disease-causing microbes. Clean water, hygiene and good sewerage systems prevent the spread of water-borne diseases such as typhoid and cholera.

Insect spread

Insects are responsible for spreading many diseases.

  1. Malaria is spread from person to person by certain species of female mosquito carrying the protozoan Plasmodium falciparum. The parasite enters the human host when an infected mosquito takes a blood meal.

  2. Bubonic plague (Black Death) is a bacterial disease of rodents caused by Yersinia pestis. It can be spread to humans and other animals by infected rat fleas. People usually get plague from being bitten by a rodent flea that is carrying the plague bacterium.

  3. Insects can also transmit pathogens to food; house flies are very good at spreading Salmonella and E.coli O157. They feed on faecal waste and transfer microbes from their feet and other body parts to food. The microbe does not invade or multiply inside the fly. House flies are very good at spreading Salmonella and E.coli O157. They feed on faecal waste and transfer microbes from their feet and other body parts to food.

Rodent Vectors

Fomites and - Microbes and Spores on Soil and Surfaces

  1. Tetanus is Deadly. A puncture wound on the finger caused by a prick from rusted iron nails, corroded surfaces like barbed wire may result in tetanus due to infection by spores of the bacterium Clostridium tetani. Such spores live mainly in soil and manure, but are also found on dirty or rusting metal objects. If untreated, tetanus (lockjaw) may be fatal.

  2. Athlete's Foot. This is a non-living object such as bedding, towels, toys and barbed wire that can carry disease-causing organisms. The fungus Trichophyton that causes athlete’s foot can be spread indirectly through towels and changing room floors. The fungus thrives in the damp warm environment found between the toes. The skin between the fourth and fifth toe is usually affected first. A flaky itchy red rash develops. The skin becomes cracked and sore and small blisters may appear. If the infection is left untreated it can spread to other parts of the body.

Polluted Water

Some diseases are caused by drinking water that is contaminated by human or animal faeces, which may contain disease-causing microbes.

This is sadly a major cause for microbial transmission by contaminated wastewater flowing from eg sewer or industrial pipes into freshwater sources.

In fact the major progress in lifespans was NOT ACHIEVED till mid 1800s when attention to proper water treatment in urban areas developed.



A cold can be caught by shaking the hand of a person who has a cold and who has just used their hand to wipe their dripping nose. The mucus from the nose will be teeming with cold virus particles such as the rhinovirus, which causes one third of colds in adults. Once the cold virus particles are on the hands of the second person they are contaminated and the virus can be transferred into their nose by their fingers.

Contaminated blood or other bodily fluids

Hepatitis B and HIV can be spread through sexual intercourse or sharing used syringe needles contaminated with infected blood.


A cold or the flu can be caught from the saliva of an infected person when you kiss them.

Air Borne spread

Measles, mumps and tuberculosis can be spread by coughing or sneezing. A cough or a sneeze can release millions of microbes into the air in droplets of mucus or saliva which can then infect somebody else if they breathe in the infected particles.

Transmission Stages, Routes and Math

Transmission stages can involve several stages.

Different pathogens have different modes of transmission. For example respiratory pathogens are usually airborne and intestinal pathogens are usually spread by water or food.

Storage in reservoirs

Leakage from Reservoir

Escape from the host or reservoir of infection (where the infectious agent normally lives and multiplies).

Transport to the new host

Entry to the new host

Escape from the new host

Epidemiology of Pandemics

Spread of Disease Vectors

The COVID-19 virus can be spread through contact with certain bodily fluids, such as droplets in a cough. It might also be caused by touching something an infected person has touched and then touching your hand to your mouth, nose, or eyes.

Math of Epidemic Spread

1. Exponential Growth period

  • Nd = number of cases on day d
  • E = average number of people that an infected person is exposed to each day
  • p = Probability of transmission

    delta Nd = Nd * E * p Nd+1 = Nd (1+ Ep) Nd = N0(1 +Ep)^d

  • So theoretically epidemics like CoV fit an exponential curve quite well

  • Plotted on an exponential Y-scale, one can fit with a regression showing actual data

2. Each disease is different in infectiousness and severity

  1. How RAPIDLY does it kill its host - if it does so, then the spread will be stunted. 2.How effective is the growth in the host is at very high rates
  2. Spread is stimulated by eg spitting, sneezing, coughing
  3. How is it transmitted - by airborne droplets, physical contact or blood/body fluids only.

  4. For example flu is highly infectious but is mild as well under 0.1% (?data) of those infected die.

  5. However CoV has 2-4% of those infected dying.

3. Saturation - Most exposures are already infected or immune

1.A good dramatization in video below shows how infected (yellow) are surrounded with already infected or now recovered people and few are new potential infectees. This indicates a severe saturation effect that greatly reduces both E even if p is the same.

  1. Isolation and Quarantine, Banning travel, Work from Home - greatly reduce E

  2. Dead and sick get removed also from general population.

4. Fear factor - people wash hands more often - reducing p

5. Vaccine development - people acquire immunity to this strain of disease

  1. Eventually, all these factors to fight disease combine to saturate - in a Logistic Curve, ie initially exponential growth, but levels off and no longer exponential growth in new cases.

  2. Inflection Point - where linear growth factor achieved and growth factor = 1.

    Growth Factor = Nd+1/Nd
  3. Growth Factor <1 reduces even below 1 and decline so Saturation to a fixed value or equilibrium.

  4. Eventual decline happens as those with no immunity die off, while hopefully vast majority are now immune.


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