As tens of thousands of people arrive in Rio de Janeiro for the Summer Olympics this week, millions of mosquitoes, some perhaps carrying the Zika virus, await them.

But what may be the best repellent around is also the hardest to get.

It comes in a greenish-gray tube. It is a sticky gel, but it has a mild, pleasant odor, which is kind of floral. It is simply called Repelente Gel, and it is produced by and for the Brazilian Army.

A military lab makes it, so good luck getting your hands on it.

The occasional civilian who has managed to find it has often ended up swearing by it. I know. I am one of them. I encountered this repellent in Haiti while on assignment for The New York Times in 2010, covering the cholera epidemic and a presidential election.

Brazilian soldiers were a large part of the United Nations peacekeeping force helping to police Haiti. One afternoon, I rode along with them on a patrol. As I got on a truck, a smiling soldier handed me a tube of his repellent and in English said, “For you, for the mosquitoes.”