He Loves Me Not
nalapaela
"The Hanahaki Disease is an illness born from one-sided love, where the patient throws up and coughs of flower petals when they suffer from one-sided love. The infection can be removed through surgery, but the feelings disappear along with the petals." (www.archiveofourown.org)
Tweek loved him, but now he does not.
Craig did not love him, but now he does.
In which Tweek becomes sick with the disease and gets it surgically removed after months of waiting for Craig to catch up, but Craig falls for him not long after he gets it removed, leaving Craig to be faced with the disease instead.