MLH told me that if he did queries involving more than one choice in a multiselect field, the results coming back were wrong. For example,
If I search for remedy = bloodletting, I get 23 results
If I search for remedy = purge, I get 52 results
Before, when I searched for remedy = bloodletting OR purge I got 0 hits.

Problem was with code that generates the where clause in the SQL query, in particular where that code has to search 2 fields. It was generating a clause like this:
((remedy1 = "bloodletting OR purge") OR (remedy2 = "bloodletting OR purge"))
and so I corrected it to create a clause like this:
(((remedy1 = "bloodletting") OR (remedy1 = "purge")) OR ((remedy2 = "bloodletting") OR (remedy2 = "purge")))

Now, when I search for remedy = bloodletting OR purge, I get 58 hits.