Contact tracing studies published in the journals Respiratory Medicine and the Annals of Internal Medicine show that asymptomatic C-19 carriers are far less contagious than those with moderate symptoms. However, a study published in the journal Nature Medicine found that pre-symptomatic C-19 carriers are highly contagious, especially in the day or two before symptoms emerge. Because there is no effective way to distinguish asymptomatic from pre-symptomatic C-19 carriers until after they are no longer infected, people who have been exposed to C-19 but don't currently have symptoms can avoid infecting others by isolating themselves. Symptoms of C-19 typically don't appear until 2 to 14 days after exposure, and C-19 tests sometimes don't detect the virus until 3 days after symptom onset.