Excerpts from an interesting article:

The first citation with the modern spelling is from a letter, written around 1655, by Dorothy Osborne to her husband, Sir William Temple: “You … never send me any of the new phrases of the town. … Pray what is meant by wellness and unwellness?”

In the 1950s, Halbert L. Dunn, chief of the National Office of Vital Statistics, resurrected “wellness” in a new sense: health care intended to prevent illness rather than cure it, according to the linguist Ben Zimmer.
Zimmer notes that language commentators belittled the new usage at first. In 1988, for example, 68 percent of the Usage Panel of the American Heritage Dictionary of the English Language disapproved of it, and a critical note was included in the 1992 edition.

“But carping over wellness faded away in the ’90s as the term gained a foothold in everyday use,” Zimmer writes, adding that American Heritage dropped the usage note in its 2000 edition.

Like it or not, the word “wellness” is alive and well today.

“A word that once sounded strange and unnecessary, even to its original boosters, has become tacitly accepted as part of our lexicon of health,” Zimmer writes. “Well, well, well.”

http://www.grammarphobia.com/blog/2012/06/wellness.html

Zimmer reports that the term wellness was initially a tough sell at the school, not least because it was associated with the “flakiness” of Mill Valley and surrounding Marin County. The Wellness Resource Center in Mill Valley had opened in 1975. The center promoted self-directed approaches to wellbeing as an alternative to the traditional illness-oriented care of physicians.

The NBC newsman Edwin Newman had televised an exposé of Marin County’s hedonistic lifestyle, which notoriously opened with a woman getting a peacock-feather massage from two nude men. The Berkeley Wellness Letter, however, managed to avoid such unseemly associations by publishing serious, evidence-based articles on health promotion, while debunking many of the holistic health fads of the day.

http://www.berkeleyside.com/2010/04/16/no-peacock-feather-ma...

This argument is getting rather silly. whether you like it or not the term is here to stay. While I am also not crazy about it in certain contexts and would certainly, most definitely avoid the overuse of "wellness" when talking about spas, beauty treatments, hotels etc for a UK audience, it does not have to be avoided completely and for the US it is absolutely fine. For UK, I would use terms like spa, wellbeing, health etc. first but wellness can be mentioned somewhere along the way, e.g.:

"The Art of Living
At Lifehouse Spa & Hotel, we consider therapeutic programmes for strengthening health and maintaining wellbeing as an integral part of our services.
We offer a simple, fresh and inspiring menu of health and wellness services that responds to our guests' needs on many levels."
http://www.lifehouse.co.uk/wellness/

"Wellness in the workplace" an article in the Guardian, if you read it you will note, however, that wellness is only mentioned once (as part of a title) in the article itself - instead they talk about "health and wellbeing"
http://www.guardian.co.uk/worklifeuk/wellness-in-the-workpla...

But:
"Last year the Department of Health and Department for Work and Pensions released details of a study commissioned by them and carried out by PWC to determine the return on investment from wellness programmes."
http://www.cipd.co.uk/NR/rdonlyres/9EC5EF61-B56E-4C70-B52C-D...

"The Alternative Health Experts
Welcome to the beginning of your journey into wellness and a totally new approach to health."
http://www.balancedwellness.co.uk/

"Queen Square Wellness Centre
Queen Square Wellness Centre is a holistic treatment centre situated in the heart of the city of Leeds.
We are owned and operated by The Faculty of Health & Social Sciences at Leeds Metropolitan University"
http://queensquarewellnesscentre.co.uk/

The list is endless: Snowsfields Wellness in London; Wellness for Men in Bristol; online wellness programme from Bupa UK; The Wellness Centre in Stevenage, Herts; The Wellness Company in Edinburgh offers an "An employee wellness programme" and "delivers health and wellbeing courses in the workplace"; Barts Health and Wellness Centre - an occupational health centre etc. etc. from Glasgow to Belfast to Brighton.

So it is not pure Denglisch. It perhaps should not be used as the catch-all phrase that it has become in German/Europe but it is well established in the US both in medicine/science and in tourism (not just in the dictionary) and as I said above is (increasingly) common in UK too.

USA examples
"DEFINITIONS OF HEALTH & WELLNESS

Wellness is first and foremost a choice to assume responsibility for the quality of your life. It begins with a conscious decision to shape a healthy lifestyle. Wellness is a mind set, a predisposition to adopt a series of key principles in varied life areas that lead to high levels of well-being and life satisfaction.
A consequence of this focus is that a wellness mind set will protect you against temptations to blame someone else, make excuses, shirk accountability, whine or wet your pants in the face of adversity. (I threw that in to help you remember this explanation.)
--Don Ardell, Ph.D., Living Well Center, University at Buffalo
http://www.livingwell.buffalo.edu/well.shtml

What is wellness?
It's a word that's used a lot these days, and I'm sure you hear it all the time. It's a buzz word almost, and traditionally, health and wellness have been thought of as the mere absence of disease and disability. This is known as the medical model, and the problem with it is that it promotes the idea that one cannot be well while living with a disability.
In recent years there has been new thinking in this area and it has led to the development of newer models of health and there are numerous different models.
Basically, the wellness model that people adhere to today moves beyond that traditional notion of health and wellness as being the mere absence of disease to the optional functioning of each individual regardless of current health status or disability...
--Carla Culley, Research & Training Center on Health, Wellness, & Disability Oregon Health Sciences University
http://www.healthwellness.org/index.htm"

from:
http://www.ilru.org/healthwellness/healthinfo/wellness-defin...

"The wellness movement began after the end of World War II largely because society’s health needs changed...

...Dunn (1977) emphasized wellness as a positive state, one that is beyond simply non-sickness, elaborating on the WHO definition by emphasizing the varying degrees of wellness and its interrelated, ever-changing aspects. He detailed the interconnected nature of wellness of the mind, body, and environment, which exists as a dynamic equilibrium as one tries to balance between each. Dunn (1977) conceptualized the dimensions of wellness fluctuating as people make active choices moving toward or away from their maximum potential...

...Rickhi and Aung (2006) believe creating wellness can mean focusing on practices that benefit one or all of the three dimensions—body, mind, and spirit. Physical wellness includes drinking water, healthy eating, healthful touch such as massage, and physical activity. Mental and spiritual wellness requires mind/body-based stress reduction programs, adapting the body to nature and being aware of the senses. Recently, Smith and Kelly (2006) have suggested that lack of community may be spurring wellness tourists to seek a sense of community within a holistic centre, a
yoga retreat, at a New Age festival, or on a pilgrimage"
http://www.geog.uvic.ca/wellness/wellness/2_DefiningWellness...

'At first glance, this mission had seemed like a breeze: a search for “wellness” — that seemingly unimpeachable state that has become as common a come-on in travel circles as “eco-friendly.” There are wellness retreats, wellness diets, wellness beauty treatments, wellness classes, wellness resorts, wellness hotels, wellness weekends and, of course, wellness experts.
“Wellness is this feeling of confidence, this feeling of vitality, this feeling of “You got this,’ ” said Dr. Jim Nicolai, the medical director of the Andrew Weil integrative wellness program at Miraval Resort and Spa, in Tucson, Ariz. “Wellness is a verb just as much as an adjective.”'
http://travel.nytimes.com/2012/12/30/travel/destination-well...