Is sad or irritable without a known trigger (no recent experience, such as being rejected by peers, caused the mood change).

For example, avoid eating in restaurants that are smoky or allow cigarette smoking - a known trigger of asthma.

Skin damage is a known trigger for psoriasis flares.

Sometimes the atria begin to fibrillate with no known trigger.

Panic attacks may also start suddenly without a known trigger.

The list of known triggers is long and varied although it's generally something that is inhaled.

It can also be idiopathic, that is have no known underlying trigger.

The attacks of involuntary movements last less than one minute and have a known trigger, usually a sudden voluntary movement.

Perhaps exposure to sunlight, he said, a known trigger for lupus, had caused a flare-up.

The return of symptoms, with the exception of known triggers, is sudden and unpredictable.