Obligate

Every host is a class

There are no character picks. The cell you are wearing is your class, and you change class by changing bodies. A neutrophil is a shotgun with a sprint. A T cell is a sniper. A macrophage is the tank: it engulfs enemies whole and heals off what it hits.

Outside a host you are a capsid: fast, unarmed, and dying. Everything you do in this game, you do to keep from being a capsid for long.

The swap is the key

Host cells are free. Immune cells have to be worn down first: shoot one below half, it starts to glow, and then you burst out of whatever you are wearing and touch it. For a second you are nothing at all. Then you are the thing that was hunting you, with its gun in your hands.

The organ will not let you skip it

Somewhere on the way to the lymph node the way on is shut, and it opens for one kind of cell: a clot only a macrophage can eat through, a plug of mucus a neutrophil dissolves, a capillary too tight for anything but a red blood cell. Find one. Become it. Come back.

The lymph node

At the end of the organ waits the dendritic cell, the immune system’s teacher. Killing it seals the node, so the game will not let you. Wear it down to the mark, leave your host, and ram it naked. If you make it, the immune system answers to you.

Your genome follows you the whole way: eighteen genes, each stackable, riding from host to host even when the body does not.