Eli5: how do scientists know that someone can spread disease before showing symptoms of the disease? For example, being able to spread the flu before any symptoms start.
In: Biology
They can figure out the incubation period in the lab with petri dish cultures and the like. Also a rough idea can be gained if you know exactly when the person got infected, e.g. When their holiday to Wuhan was or whatever
Just because you are carrying the disease does not mean it’s attacking you. Not all diseases/viruses will immediately start attacking you, some, probably most, will wait a bit. As too how scientists determine how long it takes for the virus to become active, I’ve no idea. Probably monitor samples of the virus in a lab and see how it reacts with biological material (which is also part of the process in developing a cure/vaccine/medicine to help fight the illness)
A pathogen only causes symptoms when it either negatively impacts function, or the body impacts function to respond to it. Simply because a pathogen hasn’t impacted function, to an extentent that is noticable no less, doesn’t mean that pathogen can’t be transmitted.
Take the flu for example. The flu is spread primarily through water vapor emitted from the lungs. That process can occur any time after the virus has reached the lungs and started reproducing. However, there will be a point when the viral content of your lungs is neither sufficient to cause an immune response, nor enough to cause a bad cough. During this period, the host is contagious, but not symptomatic.
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