original version of this story Appeared in Quanta Magazine.
Pathogens must overcome many things in order to make us sick. First, they must bypass natural barriers such as the skin, mucus, cilia, and stomach acid to enter the body. Then you need to reproduce it. Some bacteria and parasites can do this virtually anywhere in the body, while viruses and some other pathogens can only do this from within cells. And all the while, it must fend off attacks from the body’s immune system.
Therefore, although we are constantly bombarded with microorganisms, the number of microorganisms that enter our bodies is usually too small to penetrate our defenses. (Even in small enough doses, they may serve to remind our immune system of the presence of the pathogen, increasing the antibody response and continuing to protect us from it.)
When enough pathogens break through our defenses and start replicating, we become sick. Often this is just a numbers game. The more intruders you fight off, the more likely you are to feel sick.
How many microorganisms do we need to enter our bodies before we get sick?
This varies depending on the pathogen and is known as the “infectious dose” of the microorganism. Although usually a significant number of microorganisms is required, some microorganisms require an incredibly small number of microorganisms to initiate an infection. Take norovirus for example: Stomach bugs are notorious for spreading whenever people come into close contact and touch the same surfaces. cruise ship.That infectious dose may be as small as 18 separate viruses, is incredibly easy to send. It’s also very strong outside the body, so if an infected person exudes the virus, they can leave enough virus in their body to easily infect others, even days later.
What about the concept of “viral load”? Does it matter?
These are similar ideas, but while infectious dose refers to the number of microorganisms that cause an infection, viral load is an active measurement of infection, i.e., the number of microorganisms that are replicating within the host. The term was first introduced to the general public as part of understanding HIV/AIDS, but its use has increased since the beginning of the coronavirus pandemic.
How do researchers understand the infectious dose of microorganisms?
It’s still an inexact science. This gold standard study, called human challenge testing, involves intentionally administering the pathogen to people.Unfortunately, this approach ethically difficult Because it (obviously) carries the risk of serious illness and potential long-term complications.
So the researchers decided to expose guinea pigs, rats, mice, and ferrets depending on the pathogen. However, directly extrapolating animal doses to human doses can be difficult.
In addition, the route of infection is also important. Something that enters the bloodstream directly may require far fewer microorganisms to establish than one that enters through the mouth or lungs, for example. This is because the bloodstream allows pathogens to evade many of the host’s defenses. For this reason, for example: Risk of HIV infection Transmission through blood transfusions and needle sticks is much higher than through sexual routes.
A third way to understand infectious dose is to use observational studies. Researchers estimate numbers by observing how long it takes an infected person (especially a household member or other close contact) to become ill. As you can imagine, this is often more cumbersome and inaccurate than her two previous methods.
Why is the infectious dose of some pathogens higher or lower than others?
I’m not sure. This could be due to the way the intruder operates. The researchers are was suggested Pathogens that require direct contact with host cells tended to be more effective, so their infectious doses were much lower. However, when bacteria attack host cells indirectly (e.g., by secreting proteins that harm host cells), the secretions that modify the host can be diluted over time and space; This requires a larger amount of bacteria. This idea is 2012 survey We also looked for viruses, fungi, and parasites. However, a wider range of microorganisms requires further confirmation.
What do we know about the infectious dose of the virus that causes coronavirus?
We have learned a lot in the nearly four years since it first appeared, much of it based on animal models of infection and observational studies in humans. Most animal models require high doses of virus, between 10,000 and 1 million “plaque-forming units” (PFU), with each unit sufficient to infect and kill cells in tissue culture.However, in observational studies on humans, the infectious dose Can be approximately 100-400 PFU However, this method only provides very rough guidelines.
These studies suggest that one reason the virus is so easily transmitted is because the infectious dose of the virus is relatively low. Similar to other respiratory viruses such as respiratory syncytial virus and “common cold” coronaviruses (and lower than the infectious dose of most influenza virus strains).
