To a startling degree, many coronavirus patients are reporting similar experiences. Called hospital delirium, the phenomenon has previously been seen mostly in a subset of older patients, some of whom already had dementia, and in recent years, hospitals adopted measures to reduce it.

“All of that has been erased by Covid,” said Dr. E. Wesley Ely, co-director of the Critical Illness, Brain Dysfunction and Survivorship Center at Vanderbilt University and the Nashville Veteran’s Administration Hospital, whose team developed guidelines for hospitals to minimize delirium.

Now, the condition is bedeviling coronavirus patients of all ages with no previous cognitive impairment. Reports from hospitals and researchers suggest that about two-thirds to three-quarters of coronavirus patients in I.C.U.’s have experienced it in various ways. Some have “hyperactive delirium,” paranoid hallucinations and agitation; some have “hypoactive delirium,” internalized visions and confusion that cause patients to become withdrawn and incommunicative; and some have both.

The experiences aren’t just terrifying and disorienting. Delirium can have detrimental consequences long after it lifts, extending hospital stays, slowing recovery and increasing people’s risk of developing depression or post-traumatic stress. Previously healthy older patients with delirium can develop dementia sooner than they otherwise would have and can die earlier, researchers have found.