Hillary Clinton already had a transparency problem when she abruptly left a Sept. 11 memorial ceremony in New York on Sunday, then appeared to stumble as she was being helped into a van a few minutes later. But what appears to have been a minor episode that easily could have been explained at the time only made her credibility problem worse when she seemed to deny anything at all concerning had happened — despite millions of people having seen video of the incident online. It was the kind of unforced error the Clinton campaign repeatedly has fallen victim to this election season, and as the race heats up ahead of November's election, it's precisely the sort of mistake the candidate can't afford to keep making.

Ms. Clinton's spokesman initially told reporters the candidate became overheated after standing an hour and a half on a hot, humid day and that she had been driven to her daughter Chelsea's apartment nearby to recover. A couple of hours later, Ms. Clinton came out of her daughter's building where reporters had gathered, but when asked how she was feeling, she responded only with vague platitudes about New York's lovely weather. Not until another hour had passed did the candidate's physician reveal that Ms. Clinton had been suffering from dehydration and various allergies and that she also had been diagnosed with pneumonia the previous Friday.

Polls show that many voters remain deeply skeptical of Ms. Clinton's honesty and willingness to level with the American people. The candidate's behavior over the weekend feeds into that narrative and only adds to her credibility problem at a time when the Republican opponent, Donald Trump, has zeroed in on making Ms. Clinton's health a campaign issue by repeatedly questioning whether she has the strength or endurance to serve. Neither candidate had released much about their medical histories other than token letters from their personal physicians attesting to their good health, though both have now promised to.

We understand that Ms. Clinton is wary after decades of often unfair scrutiny, but now, with the fate of the election hanging in the balance, is not the time to argue whether the public's impression that she is evasive and untrustworthy is warranted. It is a time to combat it with transparency — just as she highlighted Mr. Trump's refusal to release his tax returns by releasing hers.

Contracting pneumonia isn't a sign that Ms. Clinton is weak or unfit for office. Though pneumonia can lead to serious problems if left untreated, it is a fairly common infectious disease that responds well to antibiotics and often does not require hospitalization. It is not a disabling illness, and there's no reason to suppose Ms. Clinton couldn't serve as president because of a temporary and treatable ailment. Had she disclosed the illness — apparently even many staffers on her campaign were unaware of it — her sudden departure from the 9/11 ceremony likely would have appeared less worrisome.

Ms. Clinton's habitual guardedness is not serving her or the public well. Incidents like the one on Sunday inevitably make her come across as a person with something to hide, and in this case it was totally unnecessary. If this campaign is waged on ideas, experience and temperament, Ms. Clinton wins. She can't afford to allow it to be about anything else. It's telling that polls show the number of voters who say they are still undecided is near record highs for this stage in a race, which reflects the public's general uneasiness with both Ms. Clinton and Mr. Trump. Ms. Clinton clearly can't afford to give her opponent any more ammunition to use against her or let the substantive issues that divide her from Mr. Trump get lost in the media storm over her stumbles.