Most travellers have notice of their dates of travel, but this may not always be the case.
Some travellers may be unsure until the last minute when they will leave or for how long they will be away.
Those who may be asked to go on short-notice business trips and those visiting sick relatives may have to travel suddenly with little or no notice.
Time should be available for seeking pre-travel health advice for those travelling to higher risk countries but not all travel agents pass this on routinely to the traveller.
It may be obvious but often forgotten that last minute bookings to Paris or New York pose very different health risks from a week in India, the Gambia or a Caribbean cruise.
Travellers making a last-minute bargain booking from a travel agent, in response to adverts or online sometimes don't even appreciate what part of the world they are going to when their bookings have been made on the basis of enticing pictures of the destination and hotel descriptions.
It can take several visits to administer the ideal course of vaccines especially for multiple destinations and more so if existing illnesses, children or pregnancy are involved.
It may take up to 4 weeks to arrange and administer full courses of vaccines.
Yellow fever vaccination certificates are required as a condition of entry into some countries and do not become valid until 10 days after vaccination if it is the first dose the traveller has received.
An invalid certificate may mean the traveller is refused entry or held in isolation or quarantine for a period.
Some vaccines require two or three doses spread over several weeks to achieve full protection from primary courses.
If a type of malaria prophylaxis has not been used previously, a trial before departure to identify those who might get side-effects is important.
Extra doses of vaccines abroad
Completing courses of vaccines abroad may be possible but difficult to ensure.
Knowing the duration of the stay abroad allows an estimate (combined with information about the accommodation, activities and lifestyle) of the possible health risks.
All of these issues are compounded if travel is to help following disasters such as earthquakes, tsunamis, civil unrest and war when the advance preparation is important - see also course 1 on volunteering and humanitarian aid