It is essential that all the members of an expedition should be re-vaccinated if they have not suffered from small-pox or been vaccinated within two years. A proper supply of vaccine should be carried on the journey. Heat and bright sunlight are very detrimental to vaccine lymph. Hence the proper transport of such lymph in hot climates is a matter of great importance and by no means always easy to arrange. Where a thermos flask is available it may be used for storing and carrying the capillary tubes, or they may be wrapped up in green leaves and inserted into a hollow bamboo, the ends of which are closed with cotton wool. _Inoculation._ Owing to the prevalence of typhoid and paratyphoid fevers in hot countries it is essential that all persons, and more especially young adults, proceeding to tropical countries should undergo preventive inoculation. Two injections of the so-called triple vaccine, at ten days’ interval, are necessary, and both should be given shortly before leaving or, if arrangements can be made, one or both may be given on ship-board. _Isolation._ On the outbreak of an infectious complaint, such as small-pox (which is very common amongst negro carriers), the sufferer should be promptly isolated, and one or more attendants should take charge of the case and not be allowed to come to camp. It is wise to select as attendants those who have either had the disease, or who are protected (as by vaccination) against it. In Nigeria it was found most convenient to build grass huts, which were burnt to the ground, together with the clothing and bedding of the patient and attendants, on the termination of the outbreak. All vessels which are not destroyed must be boiled thoroughly. The motions, etc., of patients suffering from typhoid fever, cholera and dysentery should be disinfected, preferably by boiling, or, better still, destroyed by fire. _Transmission of Infection._ The traveller should remember that of late years the rôle of the healthy human “carrier” in the transmission of many communicable diseases has been established. This is true of such diseases as enteric fever and cholera, and when engaging servants, porters, and camp followers it is important to inquire into their previous history and, if necessary, to make certain that they do not harbour the specific organisms of such diseases. DISEASES AND THEIR PREVENTION AND TREATMENT. In the tropics, as elsewhere, the diseases to which the traveller is liable are due to widely differing causes, but in hot countries climatic conditions and parasites play a predominant part. Insects are very frequently to blame for the transference of the latter from the sick to the sound, both insects which are not themselves parasitic on man, _e.g._, the mosquito and the tsetse fly, and those which make man their host, _e.g._, lice and fleas.