"There was one case," said Roger.
"Did you see it?"
"No," said Roger; "I didn't."
"I should like to see yellow fever," said Lionel simply. "I suppose there was a good deal of fuss directly this case occurred?"
"Yes," said Roger. "A gang came round at once. I think they put paraffin in the cisterns. They sealed the infected house with brown paper and fumigated it."
"And that stopped it?"
"Yes. There were no other cases."
"It's all due to a kind of mosquito," said Lionel. "The white-ribbed mosquito. He carries the organism. You put paraffin on all standing puddles and pools to prevent the mosquito's larvæ from hatching out. My old chief did a lot of work in Havana, and the West Indies, stampin' out yellow fever. It has made the Panama Canal possible."
"Are you a doctor, then, may I ask?" said Roger.
"No," said Lionel. "I do medical research work; but I don't know much about it. I never properly qualified. I'm interested in all that kind of thing."