Ds, who was dc#3 for me was like that.
His first illness was at about 8 weeks when suddenly his legs came out in what looked like huge amount of bruising. Cue blood tests.
Then at 10 weeks old he had his first ear infection. And from then on about October to May was ear infection season, which meant about every 10 days he had an infection which needed antibiotics (never cleared without them) until he had the first set of grommets at 20 months.
A week after grommets he was hospitalised with dehydration due to a vomiting bug.
Then we had the season of rashes. From about September through to January we spent about once every 3-4 weeks in A&E, sent by the GP with some sort of rash. Blood test and conclusion viral rash.
Viral rashes varied from pin pricks, through to huge 2p sized red or blue/green marks. Also had one set that started on his head and you could watch them move down his body covering it entirely.
The grommets came out, and straight back into the ear infections....
Then he added tonsilitis to the set. For these, he'd often get a viral rash. But his temperature would come up, he'd vomit, then he'd just go to sleep. He'd hardly disturb for 18 hours - when he'd wake up and be pretty much okay again...
At 8yo he had alarm bells going when he had tonsilitis and didn't pick up. he missed most of a term of school, we suspect with just post viral fatigue, but they thought it might be far worse. (and he still had ear infections!)
He's 18yo now and not ill as often. He still tends to have a temperature that shoots up with any illness and then he vomits. But he's nothing like as bad. In fact, for the first time ever I don't think he's been ill since September, even though he's at uni and mixing with lots of bugs.
When he was small I suspected it was because he got 2x bugs from his sisters. I suspect he just does tend to get ill. But also if dd1 is ill, she tends to feel a bit rotten, go to bed and recover quickly. If ds is ill, then he is not-able-to-do-anything ill.
But yes, some children are more ill than others.
Dd1 has never been to A&E and had antibiotics about 6 times in her life, mostly for toothache, except for the one bad illness she had, pneumonia where she ended up on 3 types as it was antibiotic resistant to at least one type of antibiotics.
Dd2 has been to A&E perhaps 8 times, either due to accidents or being overdramatic. Similar amounts of antibiotics to dd1.
Ds has been to A&E and had antibiotics too many times to keep track. He's also broken a bone twice and had appendicitis.