"date of illness" is correct and usable in written English.
You can use it when referring to the date on which someone began to experience symptoms or was diagnosed with a health condition. For example: I was diagnosed with the flu on April 24th, so that was the date of illness.
Overall, seven (47%) of the 15 HAstV infections and nine (37.5%) of the 24 NLV infections appeared to be nosocomially acquired based on the date of admission in hospital and the date of illness.
The earliest date of illness onset reported was 1995, and the most recent date was 2009.
Participants also contributed information on demographics, past medical history, influenza related symptoms, date of illness onset, and details on oseltamivir treatment.
Date of illness onset was based on the earliest reported symptoms, except for 3 participants (2 sero-incidence cohort participants and 1 outbreak case) without clear symptom onset dates where the date of the positive RT-PCR test was used instead.
Mean concentrations were used rather than concentrations extrapolated to the date of illness, as (1) it could not be assumed that the change in a concentration from the beginning to the end of an observation period was necessarily linear, and (2) concentrations had to be assigned to the participants who did not become ill.
The median date of illness onset was 7 May.
Second, we use the most recent published datasets of S-OIV epidemics in which the dates of illness onset for confirmed cases are known [17], [18].
Dates of illness onset were from June 24 to July 8.
Dates of illness onset ranged from March 6 to April 19.
Dates of illness onset ranged from August 3 through December 31 , 2009
This distribution and dates of illness onset were used to calculate epidemic trees for each outbreak.
