Laws Of Magic: The Law of Contagion

THE LAW OF CONTAGION

First introduced by George James Frazer in The Golden Bough, the law of contagion states that once two objects/people have been in contact, a magical link persists unless/until a magical act (such as cleansing, consecration, exorcism, cord cutting, or other act of banishing) breaks the non-material bond. This law is one of the two fundamental laws that Sympathetic magic is founded upon. 
This is a powerful law (with parallels in Quantum Physics according to Isaac Bonewits) that holds true for non-magic believers (read: muggles) as well. For example, a person who does not believe in magic or believe themselves to be 'superstitious' might still have reservations about living in a house where murders took place, or owning an item previously belonging to a person deemed to be 'evil'. 

Benefits:
Under the Law of Contagion we can assume blessings from the relics we keep of deities, and are assured of the increasing power of a holy place (such as at various stone formation, wells, and other places of natural magical power)  or magical act (such as circle casting or using well known versus/sigils) through continued similar energetic intentions put into them. 
If you are sensitive to spiritual / magical energies it can be beneficial to hold or occupy a space of the person who you wish to connect with. (ie. having an item which belonged to an ancestor you wish to commune with). 

Dangers:
You are energetically / spiritually connected to every person you have ever touched. This obviously brings a lot of baggage, and is why regular spiritual cleansing should be routinely performed (especially for sensitive people such as Empaths). It is also why many people chose to cleanse a place before moving in, to remove the energies and connections of the former occupants. 
This law also translates to "the part of the thing is the whole of the thing". This is the foundation of sympathetic magic - which means that a 'piece' (hair, nail clippings, bodily fluid, even clothing) of you, magically is you. This is how things such as poppets or voodoo dolls 'loaded' with personal effects of a person's allows a Witch to work magic over that person.  (especially spells intended to gain control over a person, including love spells). In some forms of folk magic, even a person's foot print in the sand can be taken up and magically worked - called foot track magic. 
So take care and be aware of where all your personal effects end up. You don't want them falling into the wrong hands. But if you are the one looking to do the spell casting, and have a specific target in mind, then the strongest magic would be sympathetic magic (with the strongest of links being obtained in bodily fluid, hair, and nail, clippings. )