Final answer:
The incorrect verb in the sentence is 'are', which should be replaced with 'is'.
Step-by-step explanation:
The error with subject-verb agreement in the given sentence is the verb 'are'. It should be replaced with 'is' to maintain agreement with the subject 'anyone'. The corrected sentence would be:
'Lung cancer is often regarded as a smoker's affliction but anyone exposed to secondhand smoke is at risk for the disease.'