Nick observes thickening and discoloration or the nail of the index finger. nick is most likely suffering from

Onychomycosis

Explanation:

Onychomycosis is a fungal infection of nails. It is also called as tinea unguium. The fungus invade inside the nail bed. The symptoms include discoloration of nail that is white or yellow, thickening of the nail due to fungal deposition and separation of the nail from the nail bed. The toenails and fingernails both are affected.

On the basis of the above description, onychomycosis is the infection to which Nick is suffering from.