In Plague Chess, each side starts out with an extra piece,
the "diseased" (or leper, carrier, etc.). The disease piece
carries an infectious illness that their own side is
to (resistance in a population) but the other side is
susceptible to. Enemy pieces that are adjacent to the
piece for at least one turn pick up the disease
well. Diseased enemy pieces can spread the disease to
their adjacent teammates as well. The plague's
are like so:
For every turn that a piece has the disease, the amount
possible spaces that they can move decreases. Let's use a
rook as an example. In an optimal situation, the
amount of spaces that a rook could move would be 7: a
rook can jump an entire rank or file on the 8x8 board,
counting their own occupied square. On a rook's second
"infected" turn, their longest possible move would be 6
spaces. On the next turn, the rook would only be able to
move 5 spaces. This would continue until the rook was
crippled and could only move one space. The next turn is
fatal and the rook is removed from play.
The same rules apply to bishops and queens.
Infected knights would have to have different symptoms;
perhaps they would be weakened every second turn after
infection, first losing the ability to jump other pieces
then having the extending section of their "hook"
movement grow shorter every two turns.
The king is immune to the disease because of his
attention from court medical experts (and also because
king's death would defeat the purpose of checkmate).
Pawns, being lower-class and exposed to more diseases
every day, are immune as well. Optionally, bishops could
have the ability to cure or cleanse other pieces, purging
them of the disease.
Due to the vast amount of information needed to be
remembered in this chess variant, it would be nearly
impossible to play in real life, and computer simulations
would have to be used.
I apologize in advance if this makes no sense.