I've done a search of the catalogue of the US National Archives, and as far as I can see, there were no collated statistics to indicate how many soldiers were suffering from particular conditions at a given time during the American Civil War.
That suggests that nobody saw any reason to collate those statistics (perhaps not surprising, given the contemporary understanding of the nature of diseases). The total number of soldiers on the sick list represented a reduction in effective military capacity, which was obviously important information.
They may simply not have realised the value of finding out how many soldiers contracted a particular condition at a particular time and place.
Also, as far as I can see, no modern researchers have attempted to compile aggregate statistics from the surviving records.
Records of diagnosis and treatment
As for the specific questions:
Would Northern doctors have kept detailed records on the specific diagnosis, or merely record the fact that they were sick?
Yes, the doctors kept 'detailed' records that included their medical diagnosis for individual personnel. These records were kept on Carded Medical Records.
Would they have kept records of which treatments they used on which individuals?
Carded Medical Records
The US National Archives has several series of Carded Medical Records, including:
- Carded Medical Records, Regular Army, 1821-84 (RG94, Entry 529)
- Carded Medical Records, Volunteers: Mexican and Civil Wars, 1846-65
(RG94, Entry 534)
- Carded Medical Records of Officers, 1861–1865 (RG 94, Entry 533A)
- Carded Medical Records, Marine Corps, 1821-84 (RG94, Entry 533)
- Carded Medical Records, Gunboat and Naval Service, compiled 1861-65
(RG94, Entry 536)
- Carded Medical Records of Hospital Attendants, Matrons, and Nurses,
1861-65 (RG94, Entry 535)
When researching an individual soldier, the relevant Carded Medical Record card(s) would normally be traced from the Compiled Military Service Records (CMSR).
Not all cards survive. As with CMSRs, the Carded Medical Records are
... as complete as the surviving records of an individual soldier or his unit.
The Carded Medical Record cards will normally record the diagnosis. Details of treatment may be included as remarks on the card or, in the case of soldiers who were sent to a military hospital, in the linked hospital record.
For example, cards for regular army soldiers (RG94, Entry 529) should include:
- the soldier’s name;
- nature of complaint;
- date of admittance and name of military hospital;
- date returned to duty / deserted / discharged / sent to a general hospital / furloughed / died;
- references to the original hospital record.
The record description states:
The cards cover enlisted men and non-commissioned personnel; cards for commissioned officers are filed with their military service records. Cards are arranged by number of regiment, then type of unit, and initial letter of the soldier’s surname. Regimental designations are followed by Ordnance, Engineers, Signal Corps, Scouts, and miscellaneous personnel (including recruits, prisoners, service troops, and the General Mounted Service).
I found images of some of these cards online. This shows the Carded Medical Record card of Samuel Jackson who was diagnosed with Febris Intermittens (intermittent fever) on 1 May 1863:
(Click to enlarge)
Samuel was returned to duty on 5 May 1863, but was back on the sick list suffering with diarrhoea a week later:
(Click to enlarge)
On neither occasion is there any record of the treatment that Samuel received (if any), and he wasn't sent to a hospital. From the examples of Civil War Carded Medical Record cards that I've been able to find online (which may not be a representative sample) that appears to be the rule, rather than the exception!