From my 6 original fish I bought some 6 years ago, I have 4 left, including 3 clown loaches and a denisonii barb .
One barb died soon after I bought him, he was never happy from day one, sitting in the water flow and getting emanciated over a period of 3 months. The other one got startled one night and jumped into the light fittings damaging his side, next day he got infected, lost swim bladder control and couldnt feed. I kept him alive for 6 weeks but had to subject him to the clove oil treatment since nothing I tried could fix him up.
Ive put other fish in the tank since and they have died for known reasons. Bala Shark jumped out, two mini puffers lasted no more than 3 days, and 3 tiger barbs moved from another "not well maintained" tank where their 3 other pals had already died, I suspect they were "on their way out" already after a year of stress being chased by a rainbow shark. One of them them did spawn in the new tank though!
Eventually you learn, be careful when they are new, reducing stress etc, dont have anything in the tank they can damage themselves on (infection follows), get the right fish for your tank (not puffers and bala shark), reduce stress (no rainbow sharks!) etc
I had a few things happen in the old tank where disease would kill fish and I always put this down to not changing the water frequently. Touch wood, Ive not had any diseases in my current tank for 6 years and I put this down to regular water changes, minimizing the time you got your hands in the tank, making sure stuff like gravel cleaner, bucket and stuff gets dried out after use, using Melafix after every water change etc. Cycle properly and keep your plants growing to keep ammonia out of the tank. I also vary the fish diet, always got 3 or 4 different types of food to give them. Basically trying to minimize the amount of background bacteria that is in the tank and keep the fish healthy.
In short, I would say it is normal - but it shouldnt be.