“Pre-immunity” is always a present possibility in varying degrees in everyone.
Not only does the total immune system retain readiness to produce or increase production of specific antibodies used to defend cells in the past, it is constantly employing that entire immune system army in the manner of testing them, and modifying them along the way.
Just because you never had a specific antibody to a specific pathogen in the past does not mean that your immune system is incapable of devising one when it employs its army of antibodies testing the intruder’s method of attack. In fact, if you have never had a particular bug before, but get an infection of it, what I just described is exactly what your immune system will do - try to make a set of antibodies specific for the type of attack that bug has made. MOST of the time your body succeeds and fends of the intruder and even if your system initially reacted badly you recover. When your illness becomes very serious, but you recover, most of the time what doctors have done is help your body survive the symptoms the bug produced, keeping your body strong enough to build up its antibodies until they finally succeed and you recover. In the end it is 99% of the time the fact that your immune system won the war - the doctors just kept the body strong enough to to that.
Your body encounters millions of microbes every day and there is no way for you or your body to know which ones it will or or will not encounter at any time. Your immune system MUST operate with the possibility of new encounters with totally new bugs, all the time, and still devise defenses against them. It is not strange at all that your body meets some new bug and in the process concocts some specific antibodies it never needed before. If the immune system could not do that the viral and bacterial world would have ended all mammalian development on earth long ago.
Reminds me of the incredible design by God of the body.