Should "are" be "is" here?

"A medically important group are the parasitic worms"?
Is it proper grammar? A reverse construct?

Context:

Parasites cause devastating diseases
We live in a biologically complex world, which is populated not only by humans and other large animals,but also by a plethora of other organisms, some of which are harmful or deadly to us.

A variety of parasites cause disease. A medically important group are the parasitic worms (helminths), which are estimated to afflict one third of the world’s population and are particularly prevalent in subSaharan
Africa, South Asia and Central and South America. River Blindness and Lymphatic Filariasis are two diseases caused by parasitic worms. As the name implies, River Blindness (Onchocerciasis)
ultimately leads to blindness, because of chronic inflammation in the cornea. Lymphatic Filariasis,
afflicting more than 100 million people, causes chronic swelling and leads to life-long stigmatizing and
disabling clinical symptoms, including Elephantiasis (Lymphedema) and Scrotal Hydrocele (Figure 1).
Malaria has been with humankind for as long as we know. It is a mosquito-borne disease caused by
single-cell parasites, which invade red blood cells, causing fever, and in severe cases brain damage and
death. More than 3.4 billion of the world’s most vulnerable citizens are at risk of contracting Malaria,
and each year it claims more than 450 000 lives, predominantly among children (Figure 1).

More:
http://www.nobelprize.org/nobel_prizes/medicine/laureates/2015/press.pdf