During an outbreak of human infectious disease, the numbers affected will vary depending on the disease, location, susceptible population size and the environment; the worst risk is that the infection can spread universally and become a pandemic. Biological agents can be transmitted through ingestion (accidental swallowing or via hand-to mouth contact), inhalation, sexual intercourse, through the membranes of the eyes, or through contact with broken skin. Thus, for the disease to develop, there needs to be sufficient numbers of the pathogenic micro-organisms to overcome the body’s defenses. This number differs from pathogen to pathogen and infectious doses for some pathogens are unknown, being further complicated by variation in susceptibility due to environmental/human metabolic factors.
In the 21st century, epidemics seem to spread faster and further than before. Outbreaks that were previously localized can now become pandemic very rapidly due to a series of factors: population growth, increasing urbanization, greater demand for animal protein, greater travel and connectivity between population centers, habitat loss, climate change etc. With the global population estimated to reach 9.7 billion by 2050 and with travel and trade steadily increasing, public health systems will have less time to detect and contain a pandemic before it spreads.
Public health experts believe that we are now at greater risk of experiencing large-scale outbreaks and global pandemics.
Globalization: fast and intense mobility of people, with increased transport and international travel, leads to rapid spread of infections. Thus, an individual flying from one side of the world can introduce a new disease into the other side within hours, and before even showing symptoms.
Population growth: as the population increases, so does the need for land and housing. Populations expand into previously uninhabited territories, such as forests. With new territories comes contact with new animals and, inevitably, new infections. In many affected countries, the weakness of the existing health care system prevents effective access to medical interventions, prevention of infections and control practices. Migration is also responsible for the increased risk of infections. Millions of people involved in many humanitarian emergencies, fleeing from civil unrest, political instability, conflicts, wars and natural disasters, have been uprooted from their homes and become refugees, asylum-seekers or economic migrants, and find themselves living in conditions that are often overcrowded, thereby becoming particularly vulnerable to illnesses. The conflicts and wars referred to above do not only cause civilian casualties and displacements; they destroy health care facilities exactly when and where they are most needed.
In the last decades, new infectious diseases have been recognized and some well-known diseases have appeared again. The combination of antibiotic misuse in human and veterinary medicine, antibiotic resistance as well as genetic mutation of bio-agents has caused a further rapidly emerging problem concerning the worldwide spread of infections. Viruses are infectious agents that are more likely to generate pandemics, as they are more able to adapt and to jump from one species to another.
Potentially hazardous changes are also taking place in the use of land, agricultural practices and food production, such as live poultry and animal markets and deforestation, with an increased demand for meat for human consumption – which also leads to increased contact between people and wildlife and the transmission of zoonoses (a sleeping sickness, which is caused by T brucei rhodesiense (or other) parasites which can be transmitted from animals to humans).
Evidence continues to emerge that climate change is resulting in greater numbers of heatwaves and flooding events, bringing more opportunity for waterborne diseases such as cholera and for disease vectors such as mosquitoes in new regions.
