Symbolism, Imagery, Allegory
When Sam heads to the future and Alicia jokes about having a cold, he thinks she's lost her marbles. Why would getting sick be funny? We later learn why she thinks sniffling until you're Rudolf is slapstick comedy: It's because Sam moves out of her house purportedly because of his cold. Check it out:
When I went back home, it wasn't anything to do with the fights. That's what we told ourselves, anyway. I went down with a heavy cold, and I was coughing and sneezing half the night, and I kept waking Alicia up when she needed all the sleep she could get. And she wasn't happy about me picking Roof up and passing my germs on to him either, even though her mum said it was good for his immune system. (17.54)
Something tells us this isn't just about taking some medicine and resting for a couple days. In fact, Sam never goes back to Alicia's to live after that. Sure, he might blame it on feeling under the weather, but we know that's just a cover. The cold is really just a symbol for the fact that Alicia and Sam living together is no good. They don't belong together in one house or one bed—all they do is fight. Instead of admitting this to one another, though, they blame Sam's cold. They might be parents, but that doesn't mean they're super emotionally mature yet, and blaming Sam's cold is easier than looking in the mirror, especially with a runny nose.