Dear Student,
Some bacteria are harmful to us since they cause serious damage when they enter our body. The routes through which they can enter are the various openings in our body such as the mouth, nose, eyes, ears and anus. They can also infect our skin and cause infections there. As they enter our body, their function is to reproduce and release various toxins which cause discomfort and illness in us. Diarrhoea, cold, fever etc are some of ill-effects of those bacteria. Such harmful bacteria may also cause life-threatening diseases such as tuberculosis, pneumonia, etc.
Regards