Mignon Fogarty is better known online as Grammar Girl.

A South Korean quarantine officer checks the body temperature of a passenger against possible infections of Ebola virus at the Incheon International Airport in Incheon, South Korea. (AP Photo/Yonhap, Choe Jae-koo)

Ebola and West Nile virus are capitalized. But why? Not every disease is. Here’s a quick explanation, drawn from style guides and assorted other readings:

Diseases named after regions are capitalized.

Ebola is the name of a river in Zaire, and it was near the Ebola River that the virus first caused disease in humans. Thus, the disease became known as the Ebola virus.

West Nile in West Nile virus is capitalized for a similar reason: It was first found in a patient in the West Nile district of northern Uganda.

Diseases named after people are capitalized.

Some disease names are capitalized because they are named after the person who discovered them. For example, Alzheimer’s disease is named after a German doctor named Alois Alzheimer, and Down’s syndrome is named after a British doctor named John Langdon Down.

Should disease names have apostrophes? Alzheimer’s disease versus Alzheimer disease?

Somewhat peripheral to our capitalization question: When people start considering disease names, they often wonder why some have apostrophes and some don’t, and why you sometimes see the same name written with and without an apostrophe.

You sometimes see disease names such as Alzheimer (without the apostrophe) because there is a movement to omit the apostrophe from names based on the discovering physician. Some patient advocacy groups have lobbied that the apostrophe implies the disease belongs to the physician and that such names are inappropriate.

On the other hand, the argument that an apostrophe means the doctors own the disease is linguistically simplistic, and the sentiment is not universal among advocacy groups. For example, the British Alzheimer’s Society makes its opinion clear: “Alzheimer’s is often misspelt without an apostrophe which is incorrect….”

Researchers haven’t reached a consensus about including or omitting the apostrophe — at least not one that has stuck. An article by Len Leshin (which I found through the Separated by a Common Language blog), refers to a motion made by the U.S. National Institutes of Health in 1974 that read, “The possessive form of an eponym should be discontinued, since the author neither had nor owned the disorder.”

Yet, although the apostrophe-free spelling is more common in medical literature than in general writing, the apostrophe-bearing version is still in use. Stanford ethicist Hank Greely noted in a 2011 blog post that at the time, the Alzheimer’s spelling still appeared in 15 percent of English medical articles, and Down’s appeared in 13 percent of such articles. (The Washington Post stylebook spells Alzheimer’s disease with an apostrophe but omits the apostrophe in Down syndrome.)

Most disease names aren’t capitalized.

Returning to capitalization, most disease names aren’t capitalized. They are often named based on some hallmark of the condition. Diabetes, for example, was named because of what happens to people who have the disease. They urinate a lot, so the name, based on a Latin word, means “to go through, pass through, or pass over.”

Update for our most fastidious language lovers: A reader points out that the root of “diabetes” is actually Greek (from diabainein, to cross over), though it came to English from Latin.