While our body contains many good bacteria, there are some that make you sick. They release poisons and cause the body to react to them. If many bacteria grow and the infected person is not immune, they get sick. Bacterial infections are usually treated by antibiotic drugs. Some bacterial infections can be prevented with a vaccine, which creates immunity.
Viruses are simpler in structure than bacteria.
They multiply by invading cells and taking them over to make copies of themselves. Viruses make you sick by interfering with how those cells work. Our immune system can fight most viruses. Some viruses can be treated with drugs, but the antibiotics used to treat bacterial infections don't work against viruses. There now are vaccines for some serious viral diseases.