Toxic epidermal necrolysis, a severe form of Stevens-Johnson Syndrome, has about a 30% mortality rate. I understand it to be a first world problem typically caused by taking certain strong medications. It's a severe and often deadly allergic reaction.
(I'm not going to provide links because both Wikipedia entries have disturbing photos. I've verified that I'm spelling both correctly so you can google it if you want. However, you probably would be hard pressed to find an article about either condition that didn't have similarly graphic photos.)
To clarify: epidermal means it's a skin condition. Skin conditions that kill people tend to not be pretty and the symptoms are highly visible.
"necro" -- "death"
"lysis" -- "busting open"
"epidermal" -- "skin"
"syndrome" -- "no known cause"
However, it's a little unclear why TEN is a point of comparison to Ebola, as one is a transmissible disease and one is something that happens to unlucky people typically in response to common small-molecule drugs. I guess TEN sounds like a probable side effect of antiviral immune therapy, but it's not mentioned in the article. And it seems a little tricky to separate the side effects from the disease when dealing with hemorrhagic diseases in particular
So while it is often the case that when a syndrome is initially named its cause is unknown, the two are really referring to different levels of abstraction of a disease. The syndrome is its effects, irrespective of whether we know its cause.
Edit: one usage is for a set of commonly concurrent symptoms, possibly with no known cause. Another common usage is a set of concurrent symptoms with multiple known causes (e.g. toxic shock syndrome).
Toxic epidermal necrolysis (TEN) is the most lethal on a scale of 4 related necrotic skin conditions: TEN, SJS, EMMajor, EMMinor. The medical industry sees all 4 as kind of a sliding scale, similar to how burns are classified as different degrees and can be clinically distinguished but are still burns. They are categorized based on the percentage of your skin that becomes necrotic, and additionally which tissue types are affected.
My understanding is that herpes is one of the most common causes, although maybe that's only true for EMM and not TEN/SJS. For some reason, as we age, sometimes our body changes its reaction to herpes or other infections, becoming hypersensitive, resulting in one of the 4 previously mentioned necrotic skin conditions.
I'm sharing this, because even though my symptoms were literally staring the doctors in the face, it took years before a studious dermatologist figured out I had recurrent EMMajor. If you have "cankers" or "rashes" associated with your herpes outbreaks, ask your doctor about it, even if the rashes aren't painful. Recurrent EMMajor can be life-threatening without treatment and can progress with further attacks.
https://www.nhs.uk/conditions/erythema-multiforme/ (warning: disturbing pictures)
For those bipolar folks out there, remember even lamotrigine can cause SJS (still very rare), which is why you should always ramp up the dose gradually.