The ick (properly known as "ich") is a fungus that is highly contagious and first appears in the episode "Fungus Among Us." It is a lime green-colored mulch-type substance with unknown ingredients. To get de-icked, inhabitants of Bikini Bottom had to pay Mr. Krabs five dollars to get the ick off with the help of Gary.
The marine fungus has been shown to be persistently regenerating a new amount of fungus whenever a location on the host has little fungus on it. It develops slower when growing to spread but extremely fast to replace any that is lost. As evidenced by SpongeBob, the ick pays little attention to clothes as they were clean compared to the filthy sponge. The fungus was most likely able to distinguish non-biological material from hosts. Only one method of spreading is known: physical contact. In some cases, it spreads if a small amount can go through the water onto another host.
One major symptom is an extreme amount of itch as SpongeBob irresistibly scratches his infected head. This symptom appears to take effect after a while, but SpongeBob began scratching almost immediately. Another is the presence of the green fungus.
With a stubborn structure, the fungus cannot be removed easily by scratching, soap, and many other methods. Only Gary was capable of removing the entire substance for a brief moment.
The ick is a contagious sea disease that snails like Gary find delicious. It first infects SpongeBob and makes his skin extremely itchy, but he still goes to work anyway, and ends up infecting everyone else. At the end of the episode, Gary eats the ick off everyone.