Defendants were under no duty to raise plaintiff's failure to serve a notice of claim on defendant HHC as an affirmative defense, or otherwise bring to plaintiff's attention that defendant City of New York, on whom plaintiff did serve a notice of claim, was not a proper party defendant (see, Ceely v New York City Health Hosps. Corp., 162 A.D.2d 492, 493). Plaintiff's attorney should have known that a notice of claim had to be served on HHC. Defendant's conduct in not affirmatively advising plaintiff, the appearance of an HHC attorney at the General Municipal Law § 50-h hearing, and defendants' joint participation in the litigation for years before moving to dismiss for failure to serve a notice of claim on the proper party does not give rise to an estoppel claim (see, Matter of Rieara v City of N.Y. Dept. of Parks Recreation, 156 A.D.2d 206, 207). Further, "[T]he mere fact" that both the City and HHC are represented by the Corporation Counsel "does not provide the necessary nexus to equate service of a notice of claim on one with service on the other" (Ceely v New York City Health Hosps. Corp., supra, at 493). This is not altered by the fact noted above that the City conducted a hearing pursuant to General Municipal Law § 50-h (supra) , at which plaintiff was examined by an HHC attorney (Adams v New York City Tr. Auth., 140 A.D.2d 572). Nor does General Municipal Law § 50-e (3) (c) avail plaintiff. "While a municipal corporation may, by its conduct, waive an irregularity in the notice of claim, the requirements as to the manner or time of service may not be so waived" (Adams v New York City Tr. Auth., supra, at 573). Failure to serve a necessary party is not a mere irregularity.

In any event, the failure to diagnose a medical condition does not constitute continuous treatment (see, McDermott v Torre, 56 N.Y.2d 399).

Concur — Rosenberger, J.P., Kupferman, Asch and Tom, JJ.