What determines how contagious a particular pathogen is?
In: Biology
how the pathogen is transported, how long it takes for it to not be infectious anymore, and many other factors. The ones you see with illustrations are all estimates based on the current situation of the planet. Such as how many people are vaccinated or not, or the average number of people a diseased person makes contact with.
There are many things that can affect how contagious something is.
1) How it spreads/gets into the body
Some pathogens spread through droplets, they need to be in some form of water to be able to do this. Some are much lighter and stable in dry air and so can spread via airborne. Some require that they get into your gastrointestinal tract. You might imagine that certain pathogens are able to get from person to person with more ease depending on the route they can take (ie. directly getting onto your skin through contact like how a sexually transmitted infection would be much less contagious than a pathogen that spreads via air)
2) How stable it is in the environment
More stable pathogens in the environment are more likely to infect more people. Some pathogens are not able to live outside of the body for very long, others can live for a long time. An example of this is norovirus (aka. Cruise ship virus) which can live on surfaces for many hours and is not easily killed by alcohol.
3) Virulence (aka. How fast it kills)
If a pathogens kills its host quickly it is less likely to be able to spread because it then needs to find another host to do so. That is part of the reason why we think SARS did not cause a pandemic – it was too deadly. COVID-19 on the other hand causes deadly disease, but only in a handful of people. Others seem to be asymptomatic carriers and thus, its virulence is not high.
4) Infectious dose
Some pathogens only need a small amount of exposure to cause infection. Others need a high dose of it. An example of the is Legionella, which causes a very bad pneumonia, but is not contagious between contacts because to get it, you have to be exposed to a pretty high dose of the bacteria in the form of infected water supply.
5) Population factors
Some pathogens are able to take advantage of certain groups of people due to differences in their immune systems. An example of this historically is when the settlers from the west arrived in South America. The settlers were carrying diseases that the indigenous South Americans had not been exposed to and thus were not immune. This spread easily because their bodies could not fight infection. The same applies in the modern day with vaccination; where if large numbers of people are immune to a certain pathogen they protect those who are not immune by preventing spread of the pathogen before it arrives at these select people.
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