Approximately 70% of the US population carries Polyomavirus JC, named for the initials of the patient in which it was first detected. If it passes through the blood-brain barrier it causes fatal PML (progressive multifocal leukoencephalopathy), an infection of the white matter of the brain. It is incurable, the patient always terminal.
Symptoms can include dementia, loss of muscle control, slurred speech or muteness, blindness, and others like sensory distortions. That's because it's "multifocal" meaning it can affect different parts of the brain that control different functions, and can also migrate during the disease progression, creating changing symptoms.
Fortunately the body's immune defenses normally keep JC in check. It was once common among HIV patients with their suppressed immune systems. More recent HIV meds that more effectively strengthen the immune system have greatly reduced the incidence of PML, which is classified as an AIDS disease.
My concern is that making any opening in the blood-brain barrier, even created non-evasively with ultrasound, would allow the JC virus to "piggyback" on the bubbles purposely entering the brain. Because the virus is already present in most people, waiting for the opportunity to penetrate the barrier.
But I don't know how realistic a risk that is. After all, what about people who suffer serious head injuries, like bullet wounds penetrating to the brain? Do they ever contract PML, too?
And BTW, my limited layman's knowledge of PML was acquired when my late HIV+ partner was suddenly diagnosed with it. And invariably died from it 6 weeks later in 2004, since nothing could be done to save him once the virus got inside his brain. It's still considered an opportunistic HIV disease that patients can still theoretically contract even today, especially if no HIV meds are being taken that results in a weakened immune system, although as I said its incidence has apparently become low.