Looking on Prince Vault at cancelled dates for the 70s and I noticed all of these cancelled for illness.

Date CityCountryVenueTypeComment 9 Dec. 1978 Columbia, SC USA Township Auditorium C One-off: supporting Ashford & Simpson 7 Jan. 1979 Minneapolis, MN USA Capri Theater P One-off: tentatively planned, never materialized 3rd Capri Theatre show 10 Nov. 1979 Los Angeles, CA USA MacArthur Park P One-off: canceled free KGFJ show with other bands 3 Dec. 1979 Atlanta, GA USA Capri Ballroom C Prince Tour: canceled after Prince fell ill 5 Dec. 1979 Philadelphia, PA USA Bijou C Prince Tour: canceled after Prince fell ill 6 Dec. 1979 Philadelphia, PA USA Bijou C Prince Tour: canceled after Prince fell ill 8 Dec. 1979 Washington D.C. USA Ontario Theatre C Prince Tour: canceled after Prince fell ill 9 Dec. 1979 New York, NY USA The Bottom Line C Prince Tour: canceled after Prince fell ill 10 Dec. 1979 Boston, MA USA The Paradise C Prince Tour: canceled after Prince fell ill 12 Dec. 1979 Detroit, MI USA Latin Quarter C Prince Tour: canceled after Prince fell ill 13 Dec. 1979 Chicago, IL USA Park West C Prince Tour: canceled after Prince fell ill 14 Dec. 1979 St. Louis, MO USA Ville Auditorium C Prince Tour: canceled after Prince fell ill 16 Dec. 1979 Minneapolis, MN USA Orpheum Theatre C Prince Tour: canceled after Prince fell ill 28 Dec. 1979 Nashville, TN USA Nashville Municipal Auditorium C 'Party with The Stars' w/ Bar-Kays, Cameo and Pleasure. Prince did not perform. 31 Dec. 1979 Landover, MD USA Capital Centre C 'Party with The Stars' w/ Bar-Kays, Cameo and Pleasure. Prince did not perform, was replaced by Sugar Hill Gang and Sequence

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I don't have Alex Hahn's book in front of me, but I remember some dates around that time were cancelled due to him having severe bronchitis or pneumonia.
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I would not assume that all those dates were cancelled due to illness.

Very often there are other reasons. The record label and or promoters can also be a factor, especially with those Bar-Kays shows.

I actually believe that there were shows not yet known or realized that he performed in late 1979 and early 1980 that have not been documented yet. But I'm certainly still trying.

I have found a few people that have commented that he did actually perform in Philly and Washington DC on that early first tour. But I have yet to obtain any additional proof to back those assertions, unfortunately.

Was this the year he opened for the rolling Stones? I thought he canceled the rest of his dates after he was boo'd offstage

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No, the Rolling Stones shows were in 1981.
He would have performed at several others on that tour but did pull out after the LA fiasco.

However the Controversy tour started up later on in that exact same year.