Sick people tend to become pessimistic.

Literal

Sick-person [topic-は] pessimistic [to-に] [tendency-なりがちだ].

病人 means 'sick person/patient.' 悲観的になりがち uses ~がち ('tend to'), a useful suffix meaning 'apt to, prone to.' になりがちだ means 'tends to become.' A general observation about illness and mood.