- People infected by a pathogen can transmit it to other people before they start showing signs of/realise they have the disease
- People are very mobile these days and are able to travel large distances quickly by car, train and plane
- Animals may be reservoirs/intermediate hosts of the pathogen and some animals such as birds (for avian flu) can fly long distances, carrying the pathogens between countries
- Increased population densities offer large numbers of hosts accessible to pathogens
- Healthcare workers in some countries may not have sufficient sanitary/quarantine conditions/cultural customs to isolate sick patients effectively from the community.
Q3. Explain how a disease that starts in a small population, such as a family or a small village, can spread around the world within weeks or a few months.