The human body is normally quite resilient to anything thrown its way if you are roughly healthy enough to keep living without impairment. The immune system doesn't need to know what its facing to defeat it, it just will take longer and is much worse on you as its cycles through whatever method it figures out will work.

Antibodies and built up immunity to certain sicknesses is just it specializing in knowing what will work in the future, and doing so fast enough that it doesn't need to reach Fever levels or vomit level coughing because the virus doesn't get deep enough in the first place to warrant such.

Even if it mutates, you'll be much better equipped to handle it and everything else because your immune system got a good work out to strengthen itself.

This is why its important to let kids get sick regularly as they grow. Because without doing so they will grow up incredible brittle and unable to handle the ravages that exist in the world of sickness without the resilience and personal attention a child has. This is one of the reasons why guys like our grandpas could live until 103 drinking whiskey and smoking daily, while the current generations are dying off super early after being raised in increasingly sterile environments.