The Problem is the term used for the occurrence of hauntings when ghosts and other 'unworldly beings' appeared in England. Hauntings happen only at night from 6pm to 6am.
It happened about 50-60 years ago before the events in The Screaming Staircase book. At the beginning of it, Marissa Fittes and Tom Rottwell rose to fame with their skill in handling spirits. Eventually, agencies were formed to control the Problem.
Recent discoveries and many new corporations were set up due to the Problem. These corporations, such as iron-making companies, rapier producers, and other items and objects that worked against ghosts became big and rich in the following decades.
The Problem was in the forth decade by the time Lockwood & Co. was founded.
The Problem was first brought to the eyes of the public by founders of leading agencies, Marissa Fittes, a skilled listener, and Tom Rotwell, who had the Talent of Sight. DEPRAC, or the Department of Psychic Research and Control, was founded to learn about and find new ways to control the Problem. They monitor most agencies, not including Lockwood & Co.
Companies made to defend against the Problem are rich and popular among Visitor defenses. Visitor defenses include silver, iron, and salt. Things like lavender and running water are also said to deter the otherworldly spirits, although nearly not as effective as the others. Products like Greek Fire, or Magnesium Flares, are made of almost all known and popular Visitor defenses, like salt and iron filings.
Other materialistic defenses against ghosts include: a silver chain net, a curfew to help commoners stay safe during nighttime, bright light, ghost-lamps, rapiers- the official weapon of agents, salt bombs, and a salt gun.
People (living) also defend against ghosts. People like Sensitives, Agents, and Night-watch children have a certain Talent, including Listening- being able to listen for the supernatural sounds of Visitors, Touch- the Talent of touching an object and feeling the ghost attached to it; it's death; it's intentions; and it's feelings, and Sight- being able to see the presence of otherworldly beings. Agents are people who fight and destroy the Sources of ghosts for a living. Night-watch children are children between the ages of 7 and 13 who have a Talent and defend a private area for an amount of money. Sensitives are people who, even though they have a Talent, do NOT defend against Visitors or fight them, instead they are hired to find out if a portion of land is haunted.