Advanced.
When we refer to a past event, the perfect infinitive construction is used, e.g. It is said that at least
a hundred people have been killed. - At least a hundred people are said to have been killed.
The Passive Infinitive can be used when there is no need to stress the subject, that is when we do not want to point out to the doer of an action, e.g. I should like someone to take me round the gallery. - I'd like to be taken round the gallery.
The Passive Infinitive can be similar in meaning to must and have to. It is often used for giving orders and formal instructions, e.g. One is to dissolve these tablets in water before one takes them. - These tablets are to be dissolved in water before they are taken.
This sentence falls more naturally into a passive form.