In medicine it is very unusual to find a test, be it a blood test or swab or even a scan, that is always 100% accurate. Say you definitely have an infection with COVID-19 and you take a test. Most of the time, the test will agree and confirm that you have the virus. However, many of the current tests for COVID-19 are not 100% accurate and may give a negative result even if you have the virus. So If you have symptoms (fever, cough and shortness of breath), assume you have the illness and that you are infectious to other people.

Source: https://www.nytimes.com/2020/04/01/well/live/coronavirus-symptoms-tests-false-negative.html?action=click&module=Top%20Stories&pgtype=Homepage

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