Beau's lines: These are transverse ridges or depressions in the nails that can occur after a viral illness, appearing as large pits. They are caused by a temporary cessation of nail growth due to the illness, which fits the description provided.

Nail pitting: While typically associated with conditions like psoriasis or alopecia areata, nail pitting can also occur after viral illnesses. The description of large pits without separation from the underlying nail bed could align with this condition, especially if the pits are deep and well-defined.
Onychomadesis: This condition involves the separation of the nail from the nail bed, starting at the proximal end and moving distally. However, in early stages or mild cases, it might present with nail dystrophy resembling large pits without significant separation, making it a consideration.

Nail manifestations of systemic diseases (e.g., lichen planus, dermatomyositis): Although less common, certain systemic diseases can cause nail changes that might resemble large pits after a viral illness. Missing these diagnoses could lead to delayed treatment of an underlying serious condition.
Nail changes associated with immunodeficiency: In adults, especially those with compromised immune systems, viral illnesses can trigger or exacerbate nail dystrophies. Recognizing these changes is crucial for managing the underlying condition.

Darier's disease: A rare genetic disorder that can cause nail dystrophy, including the formation of red and white longitudinal stripes and V-shaped nicks at the distal end of the nails. While not typically described as large pits, the nail changes can be varied and might fit the description in some cases.
Yellow nail syndrome: Characterized by thickened, yellowish nails that grow slowly and may be associated with systemic conditions like lymphedema. The nail changes might not exactly match the description of large pits but could be considered in the differential diagnosis due to the potential for nail dystrophy.