By 31 January, 191 adults had been treated for the virus and either discharged or died at the two hospitals. The average age of these individuals was 56, and 62 per cent were men. Around half of those treated had underlying medical conditions, most commonly diabetes and high blood pressure.

这句话来自NewScientist，句子前面说有一部分病人出院了，有一部分死亡了；后面说大约有一半人都有基础病，这里为什么用had呢？如果是病人已经死亡，而且死前疾病没有痊愈，用had我可以理解；但对于出院的人，似乎应该用have？但是似乎有无法兼顾已经死亡的病例。。。

另外，had a disease、have had a disease、had had a disease的区别是什么呢？辛苦老师解答！