用 have 还是用 having 要按疾病和语气判定,
When you are having a cold,you may have a fever or a sore throat.
句子没有错,但你也可以说
When you are have a cold,you may have a fever or a sore throat.
关键在于 when 表示病情的延续性,如果你不用 when 而用 if ,下面接 having 就不对了,
正确的句型是
If you have a cold,you may also have a fever or a sore throat.
顺便一提,例句的比较简洁的表达是
Colds are often accompanied by fever and sore throat.
或
People who have a cold may also have a fever or sore throat.
再问： have 其后接表示疾病的名词，表示某种疾病。它能不能用进行时态？
再答： 不能一概而论， 比如 diabetes 糖尿病 我们不说 having diabetes 我们习惯说 People who have diabetes People who suffer from diabetes 或 People who are suffering from diabetes 其他疾病如 AIDS cancer high blood ressure, eye diseases, intestinal diseases infectious diseases 也是这样说（通常不用 are having ）。
