I probably won't be much help but there are different ways you get sick. There are sicknesses that happen because of genetic disorders and there are illnesses that are mentally based. Then there are illnesses that happen because of bacteria or germs that get into your body that aren't supposed to be there. When you get a fever that's your body trying to fight the viruses or bacteria. And you have two kinds of blood cells. The red ones don't do anything when your sick because they just carry oxygen, but there are white ones that fight sickness. This is why we have vaccines. They contain weak versions certain bacteria or viruses so our immune system recognizes that sickness and is already prepared when an encounter occurs. This is as much as I know, but I'm only in middle school so sorry if this didn't help.
