In English grammar the category voice refers to the systematic correspondence between so called active and passive clauses. Thus, normally, a clause with a transitive verb (i.e. one that occurs with an object) can be transformed into one where the object appears as the subject, as in the following pair:
all the patients.
All the patients
Often, active clauses have a subject which refers to a 'doer', or agent (as in the example), or to an external cause (as in The wind destroyed the house.). The passive provides a means of selecting another element as the grammatical subject, while leaving the agent, cause, etc. unexpressed.
A majority of passives resemble the example above in not expressing the subject of the corresponding active clause. When necessary, it may, however, be expressed as the complement of the preposition by, as in the following example:
(1) The patients were interviewed by a team of medical students.
The 'short', agentless passive (without the by-phrase) is very common in scientific texts, probably due to its impersonal flavour. For example, in descriptions of experiments, interviews, and other kinds of data-gathering activities, the active researcher need not be explicitly mentioned. In fact, style manuals for academic writers often advice strongly against the use of personal subjects.
For more discussion of notions like impersonal style, see the following link: