They invade plants and animals and sometimes make us sick. Germs invade us because they can't live on their own. In some cases that environment is a human body. Bacteria can reproduce outside of our bodies, or within our bodies as they cause infections. But not all bacteria are bad.
Some bacteria are good for our bodies - they help keep things in balance.
Bacteria are also often used as tiny factories to produce medicines and vaccines that are genetically engineered. Viruses can't live outside of other living cells. But once they are inside the body, viruses can spread and make people sick. Viruses cause chicken pox , measles, flu , and many other diseases. Unlike other plants, fungi aren't able to produce their own food from soil, water, and air.