染病 is a separable verb (离合词) meaning 'to catch a disease' or 'to become infected'. It is formed from 染 ('to infect/get infected') and 病 ('disease/illness'). When you need to add a duration, frequency, or result complement, the two parts split: for example, 染了病 ('caught a disease'), 染上病 ('caught a disease, with a sense of contraction'), or 染过一次病 ('caught a disease once'). You cannot insert an object after the full word — instead, say 染了某种病 or use a prepositional phrase like 染上这种病.