Disease, depression, poverty, were all believed to be spirits or demons that could be threatened, persuaded, or fought. This idea is especially seen in the magic charms which were used to deal with illness in Europe's past. Grendon (1909) states of Saxon charms to deal with disease that ;
"the attempt is made to expel mischief-working demons by flattery, threat, command, or even by nauseation and physical violence, the patient's body serving as the spirit's proxy in the last two methods of treatment. In the bee charm, the evil spirits possessing the swarming insects are coaxingly addressed as sigewif ("victory-dames"), a title of honor belong ing to the Valkyries. Whether experience had taught that a soft answer turneth away the wrath even of demons, or whether the belief that a demon might be conciliated by fawning had become deeply rooted, it is certain that the coaxing treatment was applied by sorcerers, and has indeed not been entirely abandoned by professional witches, thaumaturgists, and necromancers, even at the present day. When the exorcist believed himself powerful enough to cope with the hostile spirit or conjurer, he abandoned flattery' and resorted to threats."
Romanian charms emphasized a connection between najit, demon figures eventually associated with the devil, and the diseases which infect human beings.
The Balkans had chants in which the healer would attempt to banish the evil spirits to the earth or distant and lonely lands. The healer Baba Vuka would use the following chant to cure Jaundice; "Yellow, yellow, pass out of Radisav and disappear. Disappear into the earth! Disappear under a stone! You have no place here!" (Bennett and Karras)
A Slavonian Charm to banish disease goes as follows; "Be you male or female scrofula, go you out of this man to the mountains, from the mountains to the green forest, cook without fire or water, eat without desire, shit without an ass, you, scrofula, piss up my ass. go out, go out!" (Conrad, 1990_
The person performing this charm uses forbidden words in a sacred ritual, at the end of which they call on the trinity which Conrad states "is another means of increasing the afflicted's confidence in her."
While Finland often summoned one spirit to deal with another as seen by the following chant; "Short maiden, Tuoni's girl, take from the teeth this 'cur,' this Lempo from the jaws; press down thy maladies, force down thine injuries, fling down thy filthiness into an iron baking-pan, at the end of Piru's tongs, ’mong Hiisi's coals, in the fire of the evil power. Thou'lt frizzle, Tuoni's grub, thou’lt simmer, worm of earth, thy head will be badly scorched, thy despicable tongue will swell in the iron baking-pan, at the end of Piru's tongs, ’mong Hiisi's coals, in the fire of the evil power."
Tuoni, in this charm, is the lord of the underworld and the dead in Finnish mythology, and so this poem asks Tuoni's girl, likely his daughter, to take the Lempo from his jaws. Lempo being another term for demon. Obviously, as time when on most charms and chants call on Christian figures for aid, but the general notion was the same. People asked to destroy the malady that was causing them trouble.
I've also seen efforts to defeat problems with depression in ritualistic ways, with the idea being that depression was an actual spirit which could be chased off. Indeed, nearly anything could be thought of as having a spirit that caused it. Why doesn't butter churn properly? Fairies. Why is it that you're poor? A spirit of poverty, who often appears as an old man.
This idea offers an interesting notion for both fantasy writers and game masters. After all, characters could encounter such beings. What would your character say if it met the spirit of poverty? Or perhaps one of the characters could be a witch/cunning folk whose job it is to deal with the spirits of poverty, disease, and sadness. There are stories, after all, of people wrestling with the spirits that cause disease in the herd animals, or who have to drive away the disease of an epidemic. In the last case the spirit of small pox was so powerful she often killed many of those fighting her, and even armies of shamans couldn't necessarily defeat her.
What's important to keep in mind is that certain people are essentially gifted with the ability to see the malign spirits. This 'gift' comes from spirits that seek to help humanity and so allow some people the ability to see the things that ail humanity. This could be similar to a Magic Girl story, in which the character has a familiar that aids them. It could also simply be a psychic gift that they have. Whether they appreciate this gift or not would be up to you.
In 5th Edition and Pathfinder
The ability to see the malign spirits wouldn't be a class ability, skill, or feat, but a gift given to certain adventurers, perhaps only temporarily, to help them see the malign spirits. This could be done as part of a quest in which a cunning folk or kindly fairy gives the PCs the ability to deal with the spirits that are attacking a town. Although one could come up with a specific way this worked, in lore it was often simply an ability that certain fairy like creatures and deities had.
Quest Idea
A goblin shaman spotted a pride of Red Cat Fever spirits, making their way along a human road. Realizing whatever village they entered would be easy pickings for him and his fellows, he followed them to a human village, than ran to get the rest of his goblin band. As the disease spread among the people of the village, the goblins crept into town, searching out the homes of the sick, and moving into these. For now, most of the people of the village have so much to worry about they haven't noticed the goblins flitting from house to house, taking up residence in some, and simply looting the others.
The PCs will be hired by a fey/fairy to drive away the Red Cat Fever. The goblins, however, may come as a surprise. While in town the PCs will have to deal with goblins hiding in people's homes, the Red Cat Fever Spirits, and people who attack them because the fever has taken hold.
Red Cat Fever Spirits
Red Cat Fevers are born from the bitterness of those who died in poverty. They aren't entirely undead, however, as they are a manifestation of emotions, rather than the soul of the individual. As with many such bitter spirits they seek to harm those who are poor as they were. They stalk the world in small prides, searching through villages of people for those who are weak from lack of food, or exhasted from work. The Red Cat Fevers may stalk a village for days or weeks before striking their first target. Once they do, they move quickly from target to target, until they have each caused a person to become sick.
Appearance
The Red Cat Fever appears as a tiny red lion, with a flat face, almost like theirs was smashed in, and jagged teeth that almost appear like broken glass.
Persuade and Intimidation
One can persuade a Red Cat Fever to remove their curse from a target by offering them butter, which they love but can only eat if it is given to them by a mortal being.
The Persuasion DC is 15.
One can intimidate the Red Cat Fever by calling on the spirits of the underworld, for they are afraid of entering there.
The intimidation DC is 15
Tiny Malign Spirit, chaotic evil
Armor Class 13
Hit Points 22 (5d8)
Speed 40 ft.
STR DEX CON INT WIS CHA
5 (-3) 16 (+3) 11 (+0) 10 (+0) 10 (+0) 11 (+0)
Damage Resistances acid, cold, fire, lightning, thunder; bludgeoning, piercing, and slashing from nonmagical attacks
Damage Immunities necrotic, poison
Condition Immunities charmed, exhaustion, grappled, paralyzed, petrified, poisoned, prone, restrained, unconscious
Senses darkvision 60 ft., passive Perception 10
Languages common, fiendish
Challenge 1 (200 XP)
Invisibility: The Red Cat Fever is invisible to most people, except those who have been blessed with the ability to see it, can see invisible creatures normally, or that it has chosen to attack. In order to attack someone the Red Cat Fever must make themselves visible to their target for 1 round before attacking. While visible the Red Cat Fever can remain hidden using Stealth, or as per it's favored tactic, simply attack those who are sleeping with the help of their pride.
Cling to target: The Red Cat Fever can make themselves weigh nothing, allowing them to ride on people undetected, in order to sneak into people's homes. The target of this ability might feel an unexplainable prick of pain as the Red Cat Fever grabs them with a spiritual claw.
Actions:
Life Drain. Melee Spell Attack: +4 to hit, reach 5 ft., one creature. Hit: (3d6) necrotic damage.
Disease Bred: The cat feeds off the spirit energy of those suffering the Red Cat Fever's Curse. If the target of this curse fails their first saving throw after 5 days, the Red Cat Fever gives birth to a new Red Cat Fever. This malign spirit is born at full strength. If a target dies of the fever, the Red Cat Fever gives birth to a second Red Cat Fever.
The Cat cannot move further than 50 ft. from the target while the disease runs it's course, or the target begins to recover in 2-3 days.
Red Cat Fever's Curse: If the Red Cat Fever reduces a target to 0 hit points, they can instead choose to deal no damage. The target then suffers red cat fever and forgets their encounter with the Cat. After about 1-3 days the target gains one level of exhaustion and a rash that causes them to suffer a -2 to wisdom, charisma, and intelligence checks. After an additional 1-3 days the target begins to act erratically. They attack things that aren't there, weep, laugh, and scream randomly. There is a 1 in 6 chance that they will attack any person they encounter.
The target gains a Con Save DC 12 after having the Red Cat's Fever for 5 days. If they succeed they loose the erratic behavior after their next long rest. The exhaustion and rash disappear after 3 more long rests.
If they fail they gain 1 more level of exhaustion and become even more erratic. They have a 1 in 4 chance of attacking anyone they meet.
The Red Cat Fever can only have one target cursed at a time.
After 3 more days the target gains another con saving throw. If they succeed they loose one level of exhaustion every 3 days and become less erratic after they take a long rest.
If they fail a third time the target will die within 24 hours.
References
Conrad, J. (1987). Bulgarian Magic Charms: Ritual, Form, and Content. The Slavic and East European Journal, 31(4), 548-562. doi:10.2307/307051
Grendon, F. (1909). The Anglo-Saxon Charms. The Journal of American Folklore, 22(84), 105-237. doi:10.2307/534353
Slovene Studies 1211 (1990) 55-66. , SLOVENE ORAL INCANTATIONS: TOPICS, TEXTS, AND RITUALS Joseph L. Conrad
The Oxford Handbook of Women and Gender in Medieval Europeedited by Judith M. Bennett, Ruth Mazo Karras
The Power of Words: Studies on Charms and Charming in Europeedited by James Alexander Kapal¢, ?va P¢cs, William Francis Ryan
Magic Songs of the West Finns, Vol. II, by John Abercromby, [1898]