A passive clause like I was hit by a car or Ozzie has been treated for his illness implies the existence of an active clause like A car hit me or Someone has treated Ozzie for his illness. If you cannot find an appropriate active in which the subject of the passive occurs as an object, your passive is probably not a good sentence. So Harriet has been happened to get ill is ungrammatical, since there is no grammatical sentence (Something/Someone) has happened Harriet to get ill.
In general, you want to avoid using passives unless
Examples:
Active | Full Passive | Agentless Passive |
Charley attacked Bill. | Bill was attacked by Charley. | Bill was attacked. |
The teacher gave the children some candy. | The children were given some candy by the teacher. | The children were given some candy. |
Everyone considered Mary a genius. | Mary was considered a genius by everybody. | Mary was considered a genius. |
Somebody is watching me. | I am being watched by somebody. | I am being watched. |
They have bothered her. | She has been bothered by them. | She has been bothered. |
Has this helped you? | Have you been helped by this? | Have you been helped? |