It was a contagion.

They laughed helplessly and foolishly.

Only Robert was anxious.
"I'm afraid he'll wake the house," he said, looking at the doubled up figure of Jim writhing on the grass and whooping loudly.
"Or not enough," put in Cyril Scott.

He twigged Jim's condition.
"No--no!" cried Josephine, weak with laughing in spite of herself.
"No--it's too long--I'm like to die laughing--" Jim embraced the earth in his convulsions.

Even Robert shook quite weakly with laughter.