We know that close personal contact with an infected person can lead to infection. The virus is released into the air when coughing and sneezing, but also through normal exhalation, and is absorbed by the other person through the mucous membranes in the nose and mouth and then begins to multiply there. We also know that under laboratory conditions, the virus can be detected in very small droplets (aerosols) for many hours in the air and can survive on surfaces for hours or days. However, it is still unclear whether the amount there is sufficient to lead to an infection. Other means of transmission are occasionally considered as a possibility (fecal-oral or sexual transmission), but, if they take place, should be of little importance for the course of the epidemic. There is currently no evidence of transmission via blood.
After an infection, it takes an average of about a week for the first symptoms to appear, although this time period is extremely variable and can vary from one day to one month, but for most people it takes between four and nine days.
Two factors in the course of infection and disease make control of SARS-CoV-2 so difficult:
First, not everyone who is infected develops symptoms. These asymptomatic people increasingly spread the virus because they do not go into quarantine because they don't observe any symptoms.
However, if an asymptomatic person would be notified through (digital) contact tracing, eg. through a person he infected that became symptomatic or the person who infected him, the asymptomatic person could then go into quarantine to prevent further infections and get tested to initiate further tracing.
The percentage of asymptomatic persons that has been reported in literature varies between roughly between 15% and 40%. According to a study from the village of Vò, Italy, the proportion of asymptomatic SARS-CoV-2 infections is just over 40%. In Vò almost all residents were tested for the virus and asked about symptoms twice in 12 days. In Germany, testing of a large proportion of inhabitants of the region of Heinsfeld gave a value of 23% of asymptomatic people.
The second problem in containing SARS-CoV-2 is the fact that even people who finally develop symptoms are already contagious 1 to 2.5 days before they occur, and the infection rate is even greatest during this pre-symptomatic period. However, about four days after the onset of symptoms, most people are no longer contagious.
For these reasons, extensive testing and the quick identification of possibly infected people is most important.
The tests that are currently carried out on a large scale are based on the detection of the viral genetic material, the RNA, of the virus. This can be demonstrated in samples from the mouth, nose, lungs or stool. The virus is not, or only very rarely, detectable in blood and urine.
However, the fact that the viral RNA can be detected does not automatically mean that the person is contagious, because the test cannot differentiate between living and dead viruses.
So, for example, the viral RNA in the throat is detectable longer than one is contagious. The time window for a positive RNA test from a throat swab is about two days before the onset of symptoms until about ten days after, but also varies greatly from person to person. However, almost 100% of the infections are detectable up to about 5 days after the onset of symptoms. If you test before or after this time window, you get a negative test, even though the person is or was infected. This can lead to people not being reported as infected and therefore an efficient contact tracing is not possible.