INST RATED PVT PLT DECLARED AN EMER AND LANDED AT THE NEAREST ARPT DURING AN IFR FLT DUE TO PHYSICAL INCAPACITATION.

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WHILE IN CRUISE FLT; ON IFR FLT PLAN FROM CXL TO BFL; PLT BECAME ILL. SYMPTOMS: HEADACHE; MALAISE; CRAMP. REQUESTED LOWER ALT. CTLR ADVISED MEA 10000 FT; OFFERED 8000 FT TEMPORARILY. BY THIS TIME; PIC WAS LOSING PERIPHERAL VISION. DECLARED EMER AND REQUESTED VECTORS TO NEAREST ARPT. VECTORED TO ONT VIA IMMEDIATE DSCNT AND 360 DEG TURN OVER OM. REGAINED FULL VISION AT ABOUT 6000 FT JUST BEFORE ENTERING A THIN; BROKEN CLOUD LAYER. LANDED WITHOUT FURTHER INCIDENT ON RWY 26L. UNABLE TO STAND AFTER PARKING ACFT. TAKEN BY AMBULANCE TO HOSPITAL. DIAGNOSIS WAS ACUTE GASTROENTERITIS (INTESTINAL FLU) COMPLICATED BY ALT. PAX (PIC'S WIFE) WHO IS AN M.D. CONCURS AND DEVELOPED INTESTINAL SYMPTOMS ABOUT 24 HRS LATER. PROBABLE CAUSE: CONSUMPTION OF FOOD IN MEXICO FOR 3 DAYS PRIOR TO FLT. CONSULTED FLT SURGEON; WHO AGREES. PIC HAD EXPERIENCED MINOR DIARRHEA IN THE MORNING BEFORE DEP. TOOK IMMODIUM; WHICH CLRED SYMPTOMS -- PROBABLY MASKING ONSET OF GASTRIC FLU. SUCH APPARENTLY MINOR EARLY SYMPTOMS CAN LEAD TO LATER TROUBLE -- INCLUDING INCREASED SUSCEPTIBILITY TO OXYGEN WANT AT OTHERWISE SAFE ALT. PIC RECOMMENDS PLTS RETURNING FROM MEXICO BE ALERT FOR SUCH SYMPTOMS; AND IF AT ALL POSSIBLE CARRY OXYGEN; EVEN THOUGH NOT PLANNING HIGH ALT FLT.