Of all the problems which were open to me for study, typhus was the most urgent and the most unexplored. We knew nothing of the way in which contagion spread.

My first attempts to transmit typhus to laboratory animals, including the smaller species of monkeys, had failed, as had those of my predecessors, for reasons which I can easily supply today.

Most of the doctors in the Tunisian administration, especially those in country districts, contracted typhus and approximately one third of them died of it.

Just as the only reservoir for the typhus virus in nature is provided by man, so the only vector of infection is the louse. The bite of the louse is not virulent immediately after the infecting meal. It becomes so only towards the 7th day following infection.

I was less successful in my attempts to effect preventive vaccination against typhus by using the virus and in trying to produce large quantities of serum using large animals.