I think it would be better to say, "I helped the people who HAD suffered from the disease" OR "I helped the people who HAD BEEN SUFFERING from the disease." That is, use either the tense we call the past perfect, or the tense we call the past perfect continuous. Both of those tenses are used when there were two events in the past, one of which occurred before the other. In your sentence, first people suffered; and then you helped them. The past perfect simply relates two events in time. The past perfect continuous both relates two events in time, and also puts an emphasis on the duration (length of time) of the earlier event.