Gray water is untreated waste water. You might use this water for flushing toilets
The second line of defence is a group of cells, tissues and organs that work together to protect the body. This is the immune system.
When a pathogen (disease-causing organism) invades the body, the neutrophils gather at the entry site and try to engulf it and destroy it.
A T helper cell will recognise only one pathogen. If the T helper cell recognises the invader, it will immediately clone to increase numbers.
After a few days of fighting off the pathogen, you will recover from your infection.
