Our body is under silent surveillance by a patrolling police force - the immune system. Threats to our well-being might be met with Taser-like chemicals to stun or annihilate not only trespassing organisms, but also our own cells that have turned troublemaker.

At least in the earliest stages of cancer, when the gang of rogues is small, we know that the immune police force has some success in preventing or controlling cancer because people whose immune system is missing or damaged (are immunosuppressed), are much more likely to develop cancers. Even the healthy develop potentially cancerous cells at sometime in life but the immune system clears them without us noticing.