Thinking there’s something inside you
Numbness and loss of control of one (or more) limbs
Rapid muscle atrophy
Group 2
Here's an entry about Gastar's Disease! A very natural illness caused by eating infected fish. Unless you're a merfolk, you should be safe, but it's good practice to properly cook your fish anyways. If you do come down with Gastar's Disease, you'd better act fast! Get some serum made from kelp before your tail atrophies beyond the point of no return.
A parasitic star! Controls the minds of other stars to let it inside, slowly morphs the host's body and then fuses with it. Rinse and repeat!