The most common cause of acute scrotal pain is epididymitis.[7]American College of Radiology. ACR appropriateness criteria: acute onset of scrotal pain - without trauma, without antecedent mass. 2014 (revised 2018) [internet publication].
https://acsearch.acr.org/docs/69363/Narrative
North American and European data on the incidence and prevalence of acute epididymitis are limited, as the condition is not subject to national surveillance. In the US, there are approximately 600,000 cases of epididymitis reported each year.[8]Trojian TH, Lishnak TS, Heiman D. Epididymitis and orchitis: an overview. Am Fam Physician. 2009 Apr 1;79(7):583-7.
http://www.ncbi.nlm.nih.gov/pubmed/19378875?tool=bestpractice.com
One 2005 study in Canada showed that 0.9% of men seen at an outpatient practice over a 2.5-year period presented with epididymitis.[9]Nickel JC, Teichman JM, Gregoire M, et al. Prevalence, diagnosis, characterization, and treatment of prostatitis, interstitial cystitis, and epididymitis in outpatient urological practice: the Canadian PIE Study. Urology. 2005 Nov;66(5):935-40.
http://www.ncbi.nlm.nih.gov/pubmed/16286098?tool=bestpractice.com
In a study in the UK, the reported annual incidence was 2.45 cases per 1000 men between 2003 and 2008.[10]Nicholson A, Rait G, Murray-Thomas T, et al. Management of epididymo-orchitis in primary care: results from a large UK primary care database. Br J Gen Pract. 2010 Oct;60(579):e407-22.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2944950
http://www.ncbi.nlm.nih.gov/pubmed/20883615?tool=bestpractice.com
Another study in boys, undertaken in Europe, estimated the annual incidence of epididymitis at 1.2 per 1000 male children.[11]Somekh E, Gorenstein A, Serour F. Acute epididymitis in boys: evidence of a post-infectious etiology. J Urol. 2004 Jan;171(1):391-4.
http://www.ncbi.nlm.nih.gov/pubmed/14665940?tool=bestpractice.com
Epididymitis may present at any age, with the majority of patients aged 20 to 39 years.[12]Tracy CR, Steers WD, Costabile R. Diagnosis and management of epididymitis. Urol Clin North Am. 2008 Feb;35(1):101-8;vii.
http://www.ncbi.nlm.nih.gov/pubmed/18061028?tool=bestpractice.com
The majority of cases of epididymitis in children occur around the time of puberty in early adolescence.[13]Redshaw JD, Tran TL, Wallis MC, et al. Epididymitis: a 21-year retrospective review of presentations to an outpatient urology clinic. J Urol. 2014 Oct;192(4):1203-7.
http://www.ncbi.nlm.nih.gov/pubmed/24735936?tool=bestpractice.com