Incidence details

Fever
Month	Illness episodes	Person-days at risk	Incidence	Members participating
May 2011	5	409	1.22	63
June 2011	3	425.6	0.70	43
July 2011	2	428.5	0.47	35
August 2011	4	495.7	0.81	39
September 2011	3	583.0	0.51	47
October 2011	6	535.7	1.12	36
November 2011	4	519.0	0.77	34
December 2011	6	863.4	0.69	87
January 2012	4	937.3	0.43	66
February 2012	2	913.6	0.22	58
March 2012	7	935.6	0.75	59
April 2012	4	890.3	0.45	57
May 2012	4	786.0	0.51	51
June 2012	1	740.6	0.14	46
July 2012	3	716.1	0.42	46
August 2012	2	661.3	0.30	45
September 2012	5	666.6	0.75	50
October 2012	6	646.0	0.93	52
November 2012	3	557.9	0.54	41
December 2012	3	541.3	0.55	37
January 2013	4	557.3	0.72	40
February 2013	3	557.5	0.54	39
March 2013	3	608.3	0.49	49
April 2013	3	546.0	0.55	42
May 2013	4	613.1	0.49	41

Influenza-like illness
Month	Illness episodes	Person-days at risk	Incidence	Members participating
May 2011	2	186.3	1.07	35
June 2011	1	266.3	0.38	29
July 2011	0	247.3	0.00	21
August 2011	1	254.7	0.39	20
September 2011	0	323.7	0.00	26
October 2011	0	290.3	0.00	17
November 2011	1	444.1	0.23	31
December 2011	3	876.4	0.34	86
January 2012	2	946.2	0.21	66
February 2012	1	902.5	0.11	57
March 2012	6	924.8	0.65	59
April 2012	1	892.6	0.11	56
May 2012	3	756.0	0.40	51
June 2012	1	703.7	0.14	45
July 2012	1	731.1	0.14	46
August 2012	1	662.8	0.15	45
September 2012	3	672.4	0.45	50
October 2012	5	643.8	0.78	51
November 2012	1	640.8	0.18	40
December 2012	2	537.6	0.37	36
January 2013	2	519.5	0.39	38
February 2013	1	569.1	0.18	38
March 2013	3	588.1	0.51	48
April 2013	2	526.0	0.38	42
May 2013	2	610.1	0.33	41

Common cold
Month	Illness episodes	Person-days at risk	Incidence	Members participating
May 2011	11	112.1	9.81	32
June 2011	9	165.6	5.44	25
July 2011	5	151.8	3.29	18
August 2011	2	167.2	1.20	16
September 2011	4	190.0	2.11	19
October 2011	3	193.0	1.55	15
November 2011	3	187.6	1.60	16
December 2011	11	340.7	3.23	40
January 2012	8	414.9	1.93	34
February 2012	7	362.3	1.93	27
March 2012	7	393.8	1.78	28
April 2012	7	473.9	1.48	31
May 2012	2	367.5	0.54	26
June 2012	5	321.0	1.56	23
July 2012	1	307.3	0.33	21
August 2012	1	312.5	0.32	20
September 2012	6	317.6	1.89	25
October 2012	3	231.5	1.30	21
November 2012	4	204.5	1.96	21
December 2012	4	154.3	2.59	13
January 2013	6	175.4	3.42	18
February 2013	6	236.8	2.53	20
March 2013	4	219.3	1.82	21
April 2013	4	202.7	1.97	19
May 2013	4	250.0	1.60	19

Gastroenteritis
Month	Illness episodes	Person-days at risk	Incidence	Members participating
May 2011	7	237.8	2.94	40
June 2011	3	158.5	1.89	19
July 2011	4	215.7	1.85	17
August 2011	4	286.3	1.40	23
September 2011	8	407.1	1.96	29
October 2011	8	383.7	2.09	25
November 2011	7	450.0	1.56	36
December 2011	12	785.7	1.53	88
January 2012	12	861.6	1.39	66
February 2012	14	845.9	1.66	59
March 2012	13	866.5	1.50	59
April 2012	16	812.5	1.97	57
May 2012	17	681.6	2.49	51
June 2012	11	665.5	1.65	46
July 2012	8	669.1	1.20	46
August 2012	7	606.5	1.15	45
September 2012	8	634.0	1.26	51
October 2012	8	632.9	1.26	52
November 2012	9	501.5	1.79	40
December 2012	12	485.0	2.47	37
January 2013	8	523.4	1.53	40
February 2013	4	561.5	0.71	38
March 2013	12	558.7	2.15	49
April 2013	5	524.8	0.95	42
May 2013	9	571.3	1.58	41

summary updated on June 5, 2013

Definitions

Illnesses

fever: temperature > 100.4°F/38°C or feeling feverish
influenza-like illness: fever and cough
common cold: at least two of cough, sore throat, runny/stuffed up nose, and fever (excluding influenza-like illness)
gastroenteritis: vomiting and/or diarrhea

All members were asked about fever. Until December, 2011, most members were asked either about respiratory or gastrointestinal symptoms, but not both types of symptoms. Beginning in December, 2011, nearly all members were asked about fever, cough, vomiting, and diarrhea, as well as other respiratory or gastrointestinal symptoms.

Illness episode
An illness episode is a period during which a member reported illness symptoms (for self or child) in at least one symptom update (with a recall period ranging between 20 and 72 hours). Members could have more than one illness episode during a month.

Reports of illness symptoms less than 7 days apart were considered to be part of the same illness episode, even if a member reported no corresponding symptoms in the intervening period. Reports of illness symptoms less than 14 days apart, if a member had not completed any symptom updates in the intervening period, were considered to be part of the same illness episode. If a member reported only one symptom of influenza-like illness or a common cold illness subsequent to an earlier report that fully satisfied the illness definition, the subsequent report(s) was considered to be part of the same influenza-like illness episode if it met these criteria.

Person-days at risk
Person-days at risk refer to the days a person is at risk for being ill (that is, days on which a person is not already ill, or had not been recently ill, according to the definition of illness episode). One day for 10 persons and 10 days for one person both equal 10 person-days. The vertical axis in the graph on the Results page shows the number of illnesses per 100 person-days at risk. This can be interpreted as a percentage of members who became ill on the typical day that month.

Members participating
The number of members participating is the number of members who completed one or more symptom updates that month.

Limitations
These indicidence rates are based on a small number of members for a large and diverse geographic area, and should not be treated as generalizable estimates. These rates are presented here only to illustrate the incidence measure and trends among members. As with nearly all measures of crude disease incidence in open cohorts, the measure here is based on the assumption that incidence is not associated with the frequency of members' participation (length of observation) or with the changing composition of participating members over time.

As more members join and participate regularly, we will calculate incidence rates for shorter and more recent time periods, smaller geographic areas, and additional illnesses and symptoms.