This is so true. I missed about a month of workouts earlier this year because of a horrible cold/flu that lasted for three weeks and I lost basically all the progress I'd made on my bench press over the previous six months, dropping from a 90kg 5RM back to the 70kg I started at. It's frustrating that so much time spent improving something can be lost in a relatively short space of time and I'm only now getting back to where I was before. I also lost a fair amount of muscle mass, though that wasn't quite so bad because I was still able to eat a reasonable diet when ill.
I've changed up my routine now, I got bored of the PPL split I was doing and was finding it hard to commit to the six days per week plan, so I've moved to a four day a week upper/lower split. So far I'm enjoying it more and if I wanted to move back to a six day split I could still stick with a modified version of the training plan I'm doing.