Help with query statement
You can self join the table so that on the left there are the hosts for month 1, on the right for month 9:

SELECT host, risk, plugin_id, month, year
  FROM your_table t1
  JOIN your_table t2 ON t1.month = 1 
                    AND t2.month = 9 
                    AND t1.host = t2.host 
                    AND t1.plugin_id = t2.plugin_id
 WHERE risk = 'High';


This will return the hosts where plugin_id is the same for the two months.  It is not exactly clear what the other queries should be - I imagine it is not like the plugin is the same for a given month.  Please edit your question with the clarification.