Why does people will SICK????

There are mainly 2 kinds of cause making people get sick.

1. Bacteria Infection
Bacteria are germs that multiply by splitting into two cells. Those cells keep dividing. 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.
e.g. Strep throat

2. Viruses Infection
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 are vaccines for some serious viral diseases.
e.g. cold, flu, H5N1

Other causes are body getting old or mental problem due to stress.

they had sick because they got it in their friend, and he/she don't usually
wash hand.就係咁