It’s Saturday night and you are at a restaurant full of 100 people, and unfortunately two people at the restaurant have COVID-19. A few days later 4 other people who were at the bar develop symptoms of COVID-19 (fever, cough) and are diagnosed with COVID-19.
First question: Why didn’t more people at the restaurant get the virus? Well, we know that for everyone one person who has COVID-19 they spread it to, on average, 2 more individuals [This is known as the “R not” And is written as Ro].
Second question: Okay but why only those 4 people? Well who knows, maybe some people at the bar were already immune to the virus and maybe some people didn’t stay long at the bar and thus has minimal exposure. Furthermore, maybe the 4 that got COVID19 were in very close contact with them [e.g., sitting at the same table, sharing drinks]. I mention all of this because with COVID-19 you don’t just get it from being in the same room as someone for a short period of time. In general you have to be in close contact (i.e., less than 6 feet) for at least 15 minutes to have a meaningful chance of getting it.
Moving on with this make-believe story, those 4 people who had symptoms all get tested and all of them have COVID-19. We call these people “symptomatic” because at the time [or immediately before] they were tested they had symptoms and their tests came back positive. Now the job of public health is to track down all those people who were exposed to the 4. We call this “Contact Tracing”.
Third question: Okay, so if you have symptoms of COVID-19 when you were tested and you test positive we call those people “symptomatic”? Correct. I know what you are thinking, do you really need to be a doctor to know that?!
So, you get a call from public health on Tuesday and they say, “Hey you were at a restaurant on Saturday and you were exposed to people who had COVID-19, how are you feeling?” And you say, “I feel fine and I have no symptoms.” That means, as of Tuesday, you were asymptomatic [a=no. so the word literally means "no symptoms”]. Just to be safe, you get tested for COVID-19 and it came back positive. That means you have COVID-19, but you are asymptomatic. Even though you are asymptomatic you can still spread the virus.
Now let’s say it’s Wednesday you have develop mild symptoms of COVID-19 [stuffy nose, cough]. Now we would call this “presymptomatic” [pre-symptomatic = people who seem healthy WHEN THEY TEST POSITIVE, but later develop symptoms] rather than “asymptomatic”. So we thought you were asymptomatic, but we realize that you were actually pre-symptomatic.
Remember, asymptomatic means you never had symptoms and you never develop any symptoms. I know that pre-symptomatic is a confusing term. A better name would perhaps be “delayed symptoms".