The Host

The Host is Challenge #18, starting with the items, , and the Tick. The goal is to defeat Mom. This Challenge is unlocked by default.

Difficulty

 * Isaac starts with the Tick, meaning picking up other trinkets is impossible without or.
 * It is also removable using the Match Stick.
 * ,  and Gulp! can be used to absorb the Tick.


 * The lack of Item room isn't as well compensated by the starting items when compared to the other challenges. Isaac also doesn't start with an active item.

Strategy

 * Shops can sell Mom's Purse, allowing a second trinket or Smelter allowing the tick to be smelted and new trinkets to be picked up.
 * Shops can also sell, which will make blue flies and blue spiders bigger and stronger.
 * Isaac starts with one item each for the Spider baby transformation and the Beelzebub transformation. Meaning two more items from each set will give Isaac a marked advantage. While the lack of treasure rooms somewhat limits the possibility of completely either transformation, it is still possible:
 * The aforementioned counts for  both transformation ( just the Beelzebub transformation), and is in the shop pool.
 * The only other shop item that's in the spider baby set is.
 * from Bomb Bum is the only other possible item for the set, sans the treasure pool.
 * For the Beelzebub set, the only other item in the shop pool is, which synergies well with.
 * , another Beelzebub set item, can be easily acquired by not attacking Baby Plum for about 30 seconds.
 * , another Beelzebub set item, can be won from shell games, which is likely to spawn in arcades. Arcades can only spawn on even numbered or XL floors, and only when Issac has more than 5 cents. (Having more than 2 keys means the arcades may be replaced by vaults however)
 * Key masters has more than a dozen fly items that contribute to the Beelzebub set that he could pay out with.
 * You still produce flies when firing at a lowered Hosts, so it is possible to farm flies off of them

Trivia

 * A host is an organism that is infected by a parasite, which explains why Isaac has items and trinkets relating to parasites.
 * This challenge is similar to the 'Lord of the Flies' challenge in the original game.