I have a cold= right now I am sick with a cold. I might have a cold= I think I am sick, but I am not sure. It could be allergies. I might catch a cold= usually used in a context where the speaker does not want to do an activity because they fear they might become ill. For example, if someone asked me to go outside with no jacket and it is raining or snowing, I would say no because "I might catch a cold". It's a phrase- not literal. I caught a cold and I have a cold mean the same thing. I have a high fever. = a fever is just a symptom of being sick, but it does not mean the same thing as "I have a cold." A fever can come with any sickness.