Viruses are bacteria make people sick mainly by two methods:
1. By releasing toxins (poisons)which kill off the healthy cells and allow more bacteria and virus to attack the body.
2. By taking up nutrients and energy required by the human body to exist.
Once these organisms enter the body, there can three outcomes: They are resisted and killed off by the protective systems (immune response) of the body, or the bacteria and viruses kill off the human cells causing increase in severity of the infection, or the bacteria and cells manage to survive in a chronic illness.
This is, of course, a very simple description of the process. You can read more about this here:
I hope this helps.
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