What's RSV again? The *other* winter virus that's hitting hard

It's the depths of winter, and every second person you know has a cold of some sort. Most are mild but some can hit hard - as we all know from COVID and influenza. But what about the third virus in the unholy trinity of winter bugs? RSV, or respiratory syncytial virus, is tracking a lot higher than usual for this time of year. Who's most at risk, how can you protect yourself, and how much do the numbers have to do with increased testing overall? Oh, and what does "syncytial" even mean?