- The immune system attacks bacteria / viruses once they have invaded the body
- Attacking something is a kind of method for defending against something
- So, the immune system defends the body against bacteria and viruses

- Viruses can cause disease
- Bacteria sometimes cause disease
- So, bacteria and viruses can cause disease

- The immune system defends the body against bacteria and viruses
- Bacteria and viruses can cause disease
- So, the immune system defends the body against disease

Hence, the immune system defends the body against disease
In the human body, which system functions primarily to defend the body against disease?
- Viruses can cause disease 
- Bacteria sometimes cause disease 
- The immune system attacks bacteria / viruses once they have invaded the body 
- Attacking something is a kind of method for defending against something