This probably relates to the siege of Paris in 845. The viking raid had a leader named Ragnar, so it's often with stories about the semi-legendary Ragnar Lothbrok, whether or not this is the same person. At any rate, the most straightforward account of these events can be found in the Annals of St-Bertin:

A very hard winter. In March, 120 ships of the Northmen sailed up the Seine to Paris, laying waste everything on either side and meeting not the least bit of opposition. Charles [the Bald] made efforts to offer some resistance, but realised that his men could not possibly win. So he made a deal with them: by handing over to them 7,000 lb [of silver] as a bribe, he restrained them from advancing further and persuaded them to go away. (trans. Nelson 1991)

This was a really messy moment. Charles had deployed an army against the vikings, decided he couldn't beat them, and then paid them off. This was the first time that we know of that the Franks paid tribute to viking raiders. Pretty much everyone was ticked off. Charles was angry at unreliable support, and everyone who had to payed into the tribute resented having to pay off vikings when their ruler had marched in force against them and declined to do battle. There was a lot of finger pointing, and the accounts that survive tend to point the finger at Charles.

At least some churchmen, realizing that church funds and not military leadership had turned the vikings back, decided to paint the events as a spiritual victory. A story began to circulate, in which it was none other than Saint Germain—one of the patron saints of Paris—who had turned back the vikings, not Charles. Instead of focusing on the tribute payment, these stories focused on a "miraculous" outbreak of dysentery that had compelled the vikings to retreat. According to an anonymous author from the monastery of St-Germain, the viking leader returned to Denmark and reported the incident to his king: "In the land of the Christians, the dead had more power than the living” (in christianorum terra qua fuerat, majorem haberent mortui virtutem quam viventes; my trans.). Of course, a monk on the outskirts of Paris had no way of knowing what conversations occurred in the mead halls of Denmark, but this was a sly way of asserting that St Germain was more important than King Charles without having to stand behind the claim.

A similar version of events was also copied into the Annals of Xanten, which continues the story:

Thereafter the robbers were afflicted by a terrible pestilence, during which the chief sinner among them, by the name of Reginheri [= Ragnar], who had plundered the Christians and the holy places, was struck down by the hand of God. They then took counsel and threw lots to determine from which of their gods they should seek safety; but the lots did not fall out happily, and on the advice of one of their Christian prisoners that they should cast their lot before the God of the Christians, they did so, and the lot fell happily. Then their king, by the name of Rorik [= Horik], together with all the heathen people, refrained from meat and drink for fourteen days, when the plague ceased, and they sent back all their Christian prisoners to their country. (trans. Robinson 1904)

I suspect that this Xanten account is derived from the version of St-Germain, which is probably the most original version. But I've quoted the Xanten account, since it's widely available online and in anthologies in an out-of-copyright translation, and it's the version that's best known today (even among scholars). It seems like the most likely story behind what you've heard. At any rate, this was the ultimate victory. Not only has Germain turned the vikings back from Paris, but he's cowed the Danes into submission back home, too. Take that, Charles!

Regarding the fidelity of this story, it certainly is ironic, or at least it's a story that tells us more than meets the eye. That said, we probably shouldn't dismiss it outright. It's highly likely that viking camps had frequent outbreaks of disease, and a viking siege was a particularly perilous operation. It would bring a lot of men who were used to living in rural homesteads into tight living conditions that were makeshift at best. Perhaps an outbreak was also occurring in the Frankish camps, or perhaps Charles worried that the disease would soon spread from the viking camps into Paris, or perhaps it had started within the walls of Paris.

At any rate, if there was an outbreak, Charles risked a deadly disaster among a prized city and his assembled retainers. It's not clear whether Ragnar would have felt a similar responsibility for his men, who could have easily dispersed. The outbreak of disease, whether or not St Germain had a role, could have motivated both sides to conclude a hasty settlement, and it's even plausible that the Danish king would have taken dire steps to prevent the disease from spreading into Denmark, perhaps even releasing captives who presumably carried the disease with them. While we shouldn't trust the St-Germain account of Horik's reasoning, it remains plausible that he would have connected such an outbreak as an indication of some sort of divine intervention.

TLDR You're probably referring to the viking siege of Paris in 845. There's probably an authentic account of disease contributing to the breakup of the siege, and it's plausible that divine forces were considered motivating factors among the viking forces. Christian authors certainly painted it that way.