Some people have fairly low immune systems all the time due to genetic reasons. In almost every workplace, for example, there's nearly always one person who is constantly getting colds and who is usually the first to go down with 'whatever is going around'.
Others might have had a disease, possibly in childhood, which left them with an immune system which is somewhat less than perfect.
Yet others might recently have had 'a mild touch of flu', possibly with no more than a sniffle or too. That could still be enough to leave their immune system suppressed for a few weeks afterwards.
If any of the people I've mentioned above contract Covid-19, their bodies will find it much harder to cope than the bodies of people with strong immune systems.