Disease

Historically diseases haven't spread quickly over the earth, because travel between countries was slow. Now, however, one can travel halfway around the earth in a few hours. This allows an infected person to spread a disease as if each  bacteria or virus had its own airplane ticket. Wikipedia described the transmission of contagious diseases as follows.

An infectious disease is transmitted from some source. Defining the means of transmission plays an important part in understanding the biology of an infectious agent, and in addressing the disease it causes. Transmission may occur through several different mechanisms. Respiratory diseases and meningitis are commonly acquired by contact with aerosolized droplets, spread by sneezing, coughing, talking, kissing or even singing. Gastrointestinal diseases are often acquired by ingesting contaminated food and water. Sexually transmitted diseases are acquired through contact with bodily fluids, generally as a result of sexual activity. Some infectious agents may be spread as a result of contact with a contaminated, inanimate object (known as a fomite), such as a coin passed from one person to another, while other diseases penetrate the skin directly.
Transmission of infectious diseases may also involve a vector. Vectors may be mechanical or biological. A mechanical vector picks up an infectious agent on the outside of its body and transmits it in a passive manner. An example of a mechanical vector is a housefly, which lands on cow dung, contaminating its appendages with bacteria from the feces, and then lands on food prior to consumption. The pathogen never enters the body of the fly.
In contrast, biological vectors harbor pathogens within their bodies and deliver pathogens to new hosts in an active manner, usually a bite. Biological vectors are often responsible for serious blood-borne diseases, such as malaria, viral encephalitis, Chagas disease, Lyme disease and African sleeping sickness. Biological vectors are usually, though not exclusively, arthropods, such as mosquitoes, ticks, fleas and lice. Vectors are often required in the life cycle of a pathogen. A common strategy used to control vector borne infectious diseases is to interrupt the life cycle of a pathogen by killing the vector.
The relationship between virulence and transmission is complex, and has important consequences for the long term evolution of a pathogen. Since it takes many generations for a microbe and a new host species to co-evolve, an emerging pathogen may hit its earliest victims especially hard. It is usually in the first wave of a new disease that death rates are highest. If a disease is rapidly fatal, the host may die before the microbe can get passed along to another host. However, this cost may be overwhelmed by the short term benefit of higher infectiousness if transmission is linked to virulence, as it is for instance in the case of cholera (the explosive diarrhea aids the bacterium in finding new hosts) or many respiratory infections (sneezing and coughing create infectious aerosols).

Scientists are studying the transmission of diseases, and here are links to some of their research.

On 31 March 2013, the Chinese National Health and Family Planning Commission announced human cases of novel H7N9 influenza virus infections. A group of scientists, led by Professor Chen Hualan of the Harbin Veterinary Research Institute at the Chinese Academy of Agricultural Sciences, has investigated the origins of this novel H7N9 influenza virus and published their results in Springer's open access journal Chinese Science Bulletin (SpringerOpen).

Global warming trends have a significant influence on the spread of West Nile Virus to new regions in Europe and neighboring countries, where the disease wasn't present before, according to a new study by the University of Haifa. The study was commissioned by the European Centre for Disease Prevention and Control (ECDC) in Stockholm, which belongs to the European Union. The study found that rising temperatures have a more considerable contribution than humidity, to the spread of the disease, while the effect of rain was inconclusive.

TERRIBLE new forms of infectious disease make headlines, but not at the start. Every pandemic begins small. Early indicators can be subtle and ambiguous. When the Next Big One arrives, spreading across oceans and continents like the sweep of nightfall, causing illness and fear, killing thousands or maybe millions of people, it will be signaled first by quiet, puzzling reports from faraway places — reports to which disease scientists and public health officials, but few of the rest of us, pay close attention. Such reports have been coming in recent months from two countries, China and Saudi Arabia.

Today archaeologists unearthed a 'Black Death' grave in London, containing more than a dozen skeletons of people suspected to have died from the plague. The victims are thought to have died during the 14th century and archaeologists anticipate finding many more as they excavate the site.

West Nile virus (WNV) has become endemic in North America, with cases in 2012 exceeding that of any other year. As of August 28, the United States has seen 1,590 cases, 65 deaths, and 303 viremic blood donors.

As people and consumer goods travel with increasing rapidity across the globe and to remote areas, it has become much easier for viruses to spread. In 2003, all eyes were on China, where more than 800 died at the hands of a virulent strain of pneumonia, which came to be known as severe acute respiratory syndrome (SARS). More recently, Asia (and at the back door of Europe, Turkey) was again the epicenter of a global scare as researchers isolated cases of the deadly avian influenza. In each of these cases, fears of a pandemic were sufficient to warrant global media attention and international action. But while SARS and the avian flu are prime examples of modern viruses that pose a global threat, nothing in recent history has approached the devastation of HIV and AIDS.

Certain birds, particularly water birds, act as hosts for influenza viruses by carrying the virus in their intestines and shedding it. Infected birds shed virus in saliva, nasal secretions, and feces. Susceptible birds can become infected with avian influenza virus when they have contact with contaminated nasal, respiratory, or fecal material from infected birds. Fecal-to-oral transmission is the most common mode of spread between birds.
Most often, the wild birds that are host to the virus do not get sick, but they can spread influenza to other birds. Infection with certain avian influenza A viruses (for example, some H5 and H7 strains) can cause widespread disease and death among some species of domesticated birds. (For more information, see “Low Pathogenic versus Highly Pathogenic Avian Influenza Viruses.”)

Throughout history, influenza viruses have mutated and caused pandemics or global epidemics. In 1890, an especially virulent influenza pandemic struck, killing many Americans. Those who survived that pandemic and lived to experience the 1918 pandemic tended to be less susceptible to the disease.

Working with a team of UT researchers from biology, mathematics, statistics, engineering and computing, Meyers led the development of the Texas Pandemic Flu Toolkit, a web-based service that simulates the spread of pandemic flu through the state, forecasts the number of flu hospitalizations, and determines where and when to place ventilators to minimize fatalities.