Essential oil of Tea Tree (occasionally spelled "Ti-Tree", to clearly distinguish it from the Tea bush -- Camellia sinensis -- lest there be any confusion) is now very popular and readily available, recognized by health practitioners all over the world. By "Tea Tree" is meant Melaleuca alternifolia, a tree indigenous and exclusive to Australia, especially New South Wales.

Other Melaleucas exist:

Melaleuca cajeputi ("CAJEPUT" or "CAJUPUT"): Grows wild in Malaysia, Indonesia, the Philippines, Vietnam, Java, Australia and SE Asia.

Melaleuca quinquenervia viridiflora ("NIAOULI" or "MQV"): Native to Australia, New Caledonia, and the French Pacific Islands. Its essential oil is produced mostly in Australia and Tasmania.

TEA TREE has a long history with aboriginal Aussies. By WW2, Aussie soldiers and sailors were issued Tea Tree to self-treat numerous ailments of service, from wounds to tropical infections.

TEA TREE is usually used versus yeasts/fungi, viruses, sepsis, and, perhaps most uniquely, as a very strong immune-booster, favored for its versatility and tolerability (can be used topically, undiluted, as well as internally).

NIAOULI is an excellent expectorant with anti-allergy and anti-asthma properties. It is antiseptic (as are virtually all essential oils), an endocrine tonic, and a strengthener of asthenics, among its many, many applications. It synergizes with Tea Tree, Ravensara, and Calophyllum for use on all mucous membranes, and is usually used topically.

CAJEPUT is very effective against a slightly different bacterial spectrum and is used similarly to Niaouli as well as Tea Tree, but, unlike the above 2 Melaleucas, it can be a skin-irritant.

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Now:

We move to an entirely different Genus/species: MANUKA is, often mistakenly, called "Tea Tree" by some relatively unfamiliar with Aromamedicine. Manuka (Leptospermum scoparium) is an important component of Maori natural medicine. This shrub's leaves were actually used as tea by Captain Cook when he arrived in New Zealand. Some say Manuka/Leptospermum was "The original Tea Tree". And for certain indications, it happens to be used similarly to the way Tea Tree is. However:

1. It is not predominantly Terpinene, as is Tea Tree.

2. It contains significant Geraniol and Linalol, giving it a sweet, gentle fragrance -- nice in the vaporisor -- as opposed to the more medicinal smell of the Melaleucas.

Etc.

It's an entirely different plant.

Both Melaleuca and Leptospermum belong to the Myrtaceae family (as do Eucalyptus, Clove and Myrtle, for example). And some may call Leptospermum "New Zealand Tea Tree". But Leptospermum/Manuka can be quite drying to the skin and should therefore be highly diluted in carrier oil. The Maori use it for muscular pain and rheumatism. Also: Leptospermum lacks the amazing immunity-enhancing power of Tea Tree (Melaleuca alternifolia).

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Always check your "Tea Tree" products (and ALL botanical products!) for Latin classification. It's a pretty safe bet that "Tea Tree"-inclusive products such as mouthwashes, body washes, toothpicks, etc., contain Melaleuca, rather than Leptospermum: In the US, anyway. For stimulating immunity, especially, you want Melaleuca, i.e., Australian Tea Tree (safe to use straight from the bottle). In fact, look for "Manuka", NOT "Tea Tree", if it's Leptospermum (very hard to find in the US) you want, and remember to dilute it! My guess is that most who claim "Tea Tree" is a skin-irritant for them are using Leptospermum.