just wondering if goldfish--and fish in general, on average, have a certain time of year that they are more susceptible to illnesses or diseases?

Good question, I'd like to know what ppl think here too. I think during the summer I tend to expect more problems (for me, at least) as any introduced parasites will start breeding like crazy in warmer weather. On the other hand, goldies immune system is supposingly weaker during cold weather. Maybe this depends on different places too. I also wonder about temperate countries that won't have big seasonal changes?

depending on what the disease is, it could be any time of the season. also this is more about fish that are kept outside in the elements. fungus and columnaris seems to occur in early spring. flukes, the same. bacterial infections seem to like to happen during winter and the colder months.

Every ailment is different as is every fish and their immune system capabilities. theres quite a bit more to describing when a goldfish is more succeptable to infections then just pinpointing a time of year. But if i had to pick one out of the four, id say spring.

hopefully some others have some points to add.

Paul is right, most of the trouble happens when the water warms up during spring time. Although its a lot rougher for pond fish, since the indoor fish have much more stable water conditions. I read through an interesting discussion regarding this matter on a koi forum a few years back.
During the winter the fish's immune system is compromised, and a lot of parasites can survive the colder winter temperatures. When the water warms up, unfortunately the parasites are the first ones to kick in, not the fish's immune system. That takes gradually to build up, and during this time the parasites are already working at full swing.
One other aspect is that the pond filtration during winter is either completely shut off, therefore the bacteria is compromised as well. Or, when it gets really cold, part of the beneficial bacteria dies down. With a filter that isn't working on full "bacteria capacity" yet, there can be mild ammonia problems until everything is up and working again.

personally for me i have found over the years that most of my problems are in the spring and autumn.

exactly what I wanted to know! thanks everyone!