Well again as I said:
A: You are infectious before you start showing symptoms, and studies found no difference in the contagiousness between these "presymptomatic" individuals and feeling sick.
B: Studies across the world of people who are sick report anywhere between 10-40% asymptomatic rate. Bear in mind the estimates for the asymptomatic rate of the Seasonal Flu is anywhere between 5% and 25%, so that's not an extreme outlier.
C: Some "mass testing" policies implemented in various places have found large numbers of people infected without symptoms. For instance, a number of US states have started doing tests of the entire prison population: often these have found exceptionally large outbreaks (the largest single outbreaks in the country, 1000+). Marion County, Ohio's jail briefly gave Marion County the distinction of being the most infected county in the entire country per capita (a title it has since lost to Trousedale Tennessee, in another prison-related outbreak).
Of the 2,028 who tested positive, close to 95% showed no symptoms; of all tested prisoners, the rate is even higher. Whether those are true asymptomatic cases or merely presymptomatic, the
outcome could hardly be more obvious: In a jail of 2500, 2,028 were infected.
what I've read has said that they can still test negative before symptoms appear.
As opposed to testing negative after symptoms appear? That sounds... like tests need to improve a lot, if you can have symptoms and still test negative.
Where are our bioscanners, dammit?
Bear in mind also that with how the virus test works, it tends towards false negatives. I'll quote the
Harvard link I posted a minute or two ago:
Both the saliva and swab tests work by detecting genetic material from the coronavirus. Both tests are very specific, meaning that a positive test almost always means that the person is infected with the virus. However, both tests can be negative, even if a person is proven later to be infected (known as a false negative). This is especially true for people who carry the virus but have no symptoms.
Some early reports suggest that the saliva test may have fewer false negatives than the swab test. If verified, home testing could potentially quickly ramp up the widespread testing we desperately need.
...
If a test result comes back positive, it is almost certain that the person is infected.
A negative test result is less definite. An infected person could get a so-called "false negative" test result if the swab missed the virus, for example, or because of an inadequacy of the test itself. We also don't yet know at what point during the course of illness a test becomes positive.