No one knows for certain which person was first infected.

Remember that a person can be infected without knowing it.

Symptoms may appear from a few days to several weeks after a person is first infected.

Symptoms appear about 12 days after the person is infected.

If the second test is positive, consider the person previously infected.

This test has a low chance of having a false result after the first few weeks that a person is infected.

A person can be infected with the virus and not become ill for many years.

A person is infected by tuberculosis somewhere in the world every second.

Usually this is because the person was too recently infected to have developed antibodies, which can take weeks or months.

If the antibody is found, this test can show whether a person was infected recently or in the past.