Think of the cells in your nose, throat and lungs as being like a row of houses. Once a virus gets inside, it can either hold the door open to let in other viruses, or it can nail the door shut and keep its new home to itself.
Influenza is one of the most selfish viruses around, and nearly always infects alone. Others, such as adenoviruses, seem to be more up for a house share.
If rhinovirus and Sars-CoV-2 were released at the same time, only rhinovirus is successful. If rhinovirus had a 24-hour head start then Sars-CoV-2 does not get a look in. And even when Sars-CoV-2 had 24-hours to get started, rhinovirus boots it out.
The virus that causes the common cold can effectively boot the Covid virus out of the body's cells, say researchers.