Let’s begin with sex.

As a couple finishes its business, millions of sperm begin theirs: rushing toward an egg to fertilize it. But these days, scientists say, an increasing proportion of sperm — now about 90 percent in a typical young man — are misshapen, sometimes with two heads or two tails.

Even when properly shaped, today’s sperm are often pathetic swimmers, veering like drunks or paddling crazily in circles. Sperm counts also appear to have dropped sharply in the last 75 years, in ways that affect our ability to reproduce.

“There’s been a decrease not only in sperm numbers, but also in their quality and swimming capacity, their ability to deliver the goods,” said Shanna Swan, an epidemiologist at the Icahn School of Medicine at Mount Sinai, who notes that researchers have also linked semen problems to shorter life expectancy.

Perhaps you were expecting another column about political missteps in Washington, and instead you’ve been walloped with talk of bad swimmers. Yet this isn’t just a puzzling curiosity, but is rather an urgent concern that affects reproduction, possibly even our species’ future.