Bacteria are prokaryotes. That means they do not have proper cells containing a nucleus.
A bacterium is a single celled microorganism; it has a loop of DNA (genetic material) surrounded by a cell membrane. There is no proper nucleus containing chromosomes: all the “genes” are contained on a single loop of DNA.
Many bacteria are pathogenic, i.e. they cause disease. Other bacteria are saprophytic, i.e. they cause decay. However, some saprophytic bacteria can still cause illness. e.g. Botulinism is caused by saprophytic bacteria releasing toxins into the food they are rotting. Tiny quantities of this toxin can make you seriously ill.
A few bacteria can live as pathogens or saprophytes.
Lastly there are the “good” bacteria. These bacteria live inside another living thing and give their host some benefit. There are bacteria in our intestines that produce vitamins. This is called symbiosis (living together for mutual benefit).