With the elderly, even a slight ache or fever shouldn't be taken lightly.
Tips
history
Found in Sun Zhongzhang's (孙仲章) Yuan drama 《勘头巾》: '一百日以里,但有头疼脑热,都是你' — an early record of the phrase in everyday colloquial use.
register
Colloquial and warm — typical of parents, grandparents, or family context. Covers any minor illness, not just literal headache + fever. Common collocations: 有点头疼脑热, 头疼脑热的小毛病.