I guess it really depends on you're definition to what a ghost is?
In order to gauge something to be super-natural you must first understand the natural.
I have a number of friends, actually more so older people I know who have claimed to have seen ghosts, heard ghosts or have had some kind of what they claim to be a super-natural experience.
I think people simply deem something super-natural when they themselves cannot explain it, then usually with a little imagination the story becomes more fantastical.
Infra-sound is a natural phenomenon which can cause unusual experiences, this would explain a number of events, but of course not all.
We have the individual, the experience and the environment.
Infra-sounds shows that something environmental can have an effect on the individual.
In many cases you then have to look at the individual, do they suffer from any mental disorders? anything that can cause unusual feelings? anything that can cause strange sensations or even hallucination? or something as simple as being over tired or even over stimulated?
There is an experience known as "the hag" which people all over the world have experienced. The experience usually is something that happens when the individual is in bed or laying down, the experience can include seeing ghostly apparitions and a feeling of being held.
In short the explanation is quite simple:
The brain operates differently when awake to when sleeping, when we sleep a chemical in the body is release to paralyse or disconnect the thoughts / dreams to the rest of the body. However the chemical sometimes is not released properly when we sleep, this is why sometimes as we dream we jump in our sleep usually waking us up completely.
The opposite can also happen when we wake and have the sensation of not being able to move, commonly known as sleep paralysis.
When we are tried the brain can take little nano sleeps, during this time it is almost like sleeping and being awake at the same time, where dreams can almost manifest when being partially awake.
This has explained a lot of so called ghostly apparitions more so the ones involving family members or people we actually know.
Usually people who claim to have seen ghosts are people who want to see ghosts. Seeing a ghost is no different than saying you have just seen flying aliens by claiming you just saw a UFO!