语法是没问题，但这种表达方式比较别扭。翻译是：感冒是最常见的病，肺炎常常被误诊为感冒。建议用这种方式表述，更加清晰直接：Pneumonia was often misdiagnosed as cold, which was the most common disease.
感冒是最常见的疾病，也是肺炎常常被误诊成的那个疾病
