Database search systems usually follow an order of precedence when processing a search where there are two or more operators.

Databases process the search from left to right, but some evaluate AND operators first, some evaluate OR operators first.
This means your search terms may not be interpreted as you expected.

You can specify the evaluation order of your search terms by using nesting operators, or parentheses. When you nest entries, the search system evaluates the search terms inside the parentheses first, then merges the result with the search terms outside the parenthesis.

allergens AND chronic asthma OR acute asthma AND children or infants (result: 10000 records from Medline)

allergens AND (chronic asthma OR acute asthma)

(children OR infants) AND Search 1 (result: 18 records from Medline)