Your immune system is a large network of organs, white blood cells and proteins that protects you from germs and other substances that can make you sick. It also helps your body heal from infection and disease.

Immunity is protection against disease that you either have naturally or acquire through vaccination. Natural immunity, or innate immunity, is the immune response that your body has at birth to protect itself against any organism that doesn’t belong in your body. It works right away, without any prior training to tell the difference between cells that are yours and those that aren’t. This is what enables the rapid, automatic response to bacteria that enter your skin through a small cut, or to a virus that infects you with influenza or chickenpox.

The innate immune system keeps a record of every microbe it defeats, in types of white blood cells called memory cells (B-lymphocytes and T-lymphocytes). If the same type of bacteria or virus invades again, these cells recognize them immediately and destroy them before they can multiply and make you sick. This type of immunity lasts a lifetime unless it is disrupted by things like stress or illness.

Some infections, such as measles or influenza, spread easily from person to person through coughs and sneezes. These pathogens can be eradicated in a community with herd immunity, when enough people become immune to stop the disease from spreading. Others spread more slowly or have a different mechanism for transmission, such as Ebola.