病 vs 疾病

1) 病 (n.) means "a particular disease", e.g. heart disease, neuropathy, etc.

2) 病 (v.) means "to be ill". 疾(n.) is a general term for the ailment, which is more formal and can be used figuratively.

3) In the examples above, 病 is a noun in ①, a verb in ③, in ② it can either be understood as a verb or a noun.

1) One can usually say 一种疾病 (a kind of illness) or 某种疾病 (an illness) which sounds rather formal. When it is used figuratively, one can say in example ⑤.

2) 有病 (have an illness), 得病 (have got illness), 患病 (have got illness), 治病 (cure the illness), etc. are all commonly used phrases, where 疾病 is not used.