Two changes to make.
1) Since you're using .php, you can save youself some trouble when using " as your string wrapper.
$sql = "SELECT * FROM add_doctor WHERE city='".$city."' AND treatment ='".$treatment."'";
Could be written:
$sql = "SELECT * FROM add_doctor WHERE city='$city' AND treatment ='$treatment'";
The php parser will replace the $ variable at run-time.
Now, to your query. Rewrite to something like:
$sql = "SELECT * FROM add_doctor WHERE city='$city' AND treatment LIKE '%$treatment%'";
With the '%' wild-cards wrapping your diseases. This will match any string that contains the value in $treatment regardless of what surrounds it.
As noted in first comment, the design for the disease field is very bad.