The Disease (The Enemy)

The disease, also known as the sickness, is the cause of the disaster in the events of the The Enemy series. It affects people roughly over the age of fourteen (or sixteen if reading the US version). It is unknown for certain how the sickness works but, it's likely that it is a parasitic infection. Children under sixteen are not infected because they were not born when the parasites were in their "spore" stage and spread across the world.

In When the Cold Breeze Blows Away, the South Vietnamese Umbrella Forces upgraded this by mutating this disease with WhiteKnight Virus into Kẻ Thù, which infects most people at the ages of sixteen (except those who are immunized and everyone else living in South Vietnam) in hopes that young children can fend for themselves, as ordered by Nguyễn Văn Thiệu.

It was eventually eradicated by the Grownup Union in the Offensive Uprising of London.