Bacterial illnesses are treated via antibiotics, which block the viral processes bacteria need to live and end up killing it off, thereby ending the illness. Human cells are not affected, but if the antibiotics aren’t taken for the full amount of time required, antibiotic resistent bacteria can develop, which can be exponentially harmful to the body. For viruses you cannot use antibiotics, you have to treat the symptoms and hope that they body’s system can kill off the virus. The virus can harm and kill some healthy human cells as it grows and is fought against, i don’t know the extent well enough.