I think it is very plausible that boils were more common among Americans during the Depression than they are now, although I am unaware of any data supporting this supposition.
Assuming it is true, I can’t tell you what accounts for the relatively lower prevalence of boils now.
Why some people get boils some times is just not all that clear. The best study of chronic furunculosis (recurrent boils) that I ever read was by some U.S. Army dermatologists working in VietNam during the war. Boils and other skin conditions were a serious problem for the army since they made servicemen unfit for duty. After a lot of study about all the dermatologists could conclude was that the grunts out in the field walking through rice paddies in their combat boots got a lot more boils than did the clerks, technicians and such working well behind the lines.
It seems that spending time in dirty, sweaty clothes makes one more prone to recurrent boils. I suspect that more people spent more time in dirty, sweaty clothes during the Depression than they do now. I suspect that the ubiquity of washing machines and air conditioning has a lot to do with making boils relatively scarce.
But other factors are involved in causing boils. Evidently, some strains of S. aureus are more likley to cause boils than others since boils occasionally occur in outbreaks among people who associate closely with one another. Here is an excellent article from the 80’s:
Sosin DM, Gunn RA, Ford WL, Skaggs JW. “An outbreak of furunculosis among high school athletes.” Am J Sports Med 1989 Nov-Dec;17(6):828-32.
“Furuncles (boils) are common among teenagers; however, few outbreaks have been documented. We investigated an outbreak of furuncles that occurred among male athletes of a Kentucky high school during the 1986 to 1987 school year. The overall attack rate was 25% (31/124). The risk of developing a furuncle increased two to three times in those who had skin injury. Athletes who sustained abrasions more than twice per week (P less than 0.01), who had a cut that required bandaging (P = 0.01), or had an unspecified injury causing a missed practice or game (P = 0.04) were at increased risk. The risk of developing furunculosis did not appear to be related to contact with formites, but rather, to contact with furuncles. Although athletes shared common areas (showers, locker rooms, practice areas, the attack rates for varsity football (36%) and varsity basketball (33%) were four times greater than for nonvarsity teams (P less than 0.01). Players who had a friend with a furuncle were more than twice as likely to also have had a furuncle (P less than 0.01). Exposure to furuncles appeared to increase the risk of furunculosis independently of reported skin injury. Control and prevention should, therefore, focus on both reducing skin injury and reducing exposure to furuncles, rather than attempting to sterilize inanimate objects.”
We now have antibiotics that we didn’t have during the depression that we can use to control outbreaks of particularly “bad” strains of S. aureus. This probably limits the size of outbreaks of boils and in that manner makes boils less common.
The most recent investigation of the epidemiology of boils that I could find was published in April of this year:
Landen MG, McCumber BJ, Asam ED, Egeland GM. “Outbreak of boils in an Alaskan village: a case-control study” West J Med 2000 Apr;172(4):235-9
“OBJECTIVE: To determine whether taking steam baths was associated with furunculosis and to evaluate possible risk factors for the occurrence of boils during a large outbreak in Alaska. DESIGN: A cohort study of village residents, a case-control study, and assessment of environmental cultures taken from steam baths.
SETTING: Village in southwestern Alaska
PARTICIPANTS: 1 adult member from 77 of the 92 households in the village was interviewed; 115 residents with at least one boil occurring between January 1 and December 12, 1996 were considered to be cases; 209 residents without a boil acted as the control group. All 459 village residents were included in the cohort study.
MAIN OUTCOME MEASURE: Rate of infection among all residents and residents who regularly took steam baths, risk factors for infection, and relative risk of infection.
RESULTS: 115 people (25%) had had at least one boil. Men were more likely to have had a boil than women (relative risk 1.5; 95% confidence interval 1.1 to 2.2). The highest rate of infection was among people ages 25-34 years (32/76; 42%). No children younger than 2 years had had boils. Boils were associated with using a steam bath (odds ratio 8.1; 3.3 to 20.1). Among those who used a steam bath, the likelihood of developing boils was reduced by routinely sitting on a towel while bathing, which women were more likely to do, and bathing with fewer than 8 people. Of the 93 samples taken from steam baths, one Staphylococcus aureus isolate was obtained from a bench in an outer dressing room.
CONCLUSION: Using a steam bath was associated with developing boils in this outbreak in a village in Alaska. People should be advised to sit on towels while using steam baths.”
I doubt, however, that any excess of boils during the Depression was due to more people sitting in steam baths without towels under their heinies.