Summary
Highlights
Human actions significantly contribute to the evolution and spread of infectious diseases. HIV evolved from SIV in chimpanzees, spread to humans through bushmeat hunting, and then amplified through practices like shared needles and blood transfusions. The influenza virus, originating in ducks, amplifies in chicken farms due to dense populations before spreading to humans. The monkeypox outbreak in the US in 2003 was due to smuggled Gambian rats infecting prairie dogs as pets.
Historical and modern travel methods have dramatically impacted disease transmission. Early long-distance travel by foot limited pathogen spread, but ships allowed more rapid transmission, as seen with smallpox and measles in the Americas. Today, air travel accelerates global spread, as demonstrated by the rapid dissemination of COVID-19. The SARS outbreak in 2003 exemplified this, with an infected doctor attending a conference in Hong Kong leading to outbreaks across Asia, Europe, and North America.
The 2003 SARS outbreak, originating from bats and transmitted to humans via civets, spread globally from a single event at a Hong Kong hotel. An infected doctor attending a conference unknowingly spread the virus to other attendees who then flew to different countries, causing multiple outbreaks. This event underscored the speed at which modern travel can disseminate pathogens. Although aggressive, SARS was contained through public health measures like those implemented by the WHO, which restricted travel to affected areas like Toronto.
New viruses periodically emerge to infect humans. HIV, unknown before the 1980s, drastically changed medical history. Coronaviruses like SARS (2003), MERS (2012), and COVID-19 are other examples of new aggressive or highly contagious pathogens that have emerged and spread globally.
Diseases normally localized in one region can emerge in new locations. The West Nile virus, previously found in the Middle East and Africa, appeared in North America in 1999, likely transported via mosquitoes in recycled tires. This highlights how globalization and human activities facilitate the spread of diseases.
In the late 19th century, infectious diseases were a leading cause of death, accounting for 60% of fatalities. Child mortality was high, with a significant percentage of children dying early due to infections. However, significant progress has been made since then, with infectious disease deaths in high-income countries now around 6%.
Key factors in combating infectious diseases include improved sanitation in hospitals (doctors washing hands), better water sanitation (separating sewage and drinking water), and the development of vaccines, particularly against smallpox and later other viruses. Antibiotics, developed in the mid-20th century, also played a crucial role in treating bacterial infections.
Despite progress, success led to complacency. In the 1960s, there was an optimistic view that the war against infectious diseases was nearly over, culminating in the U.S. Surgeon General's statement in 1969 to 'close the book on infectious diseases'. This led to a significant reduction in research funding for infectious diseases, with many scientists shifting to other fields like cancer research. This arrogance underestimated the persistent threat of microbes.
A major challenge today is the emergence of drug-resistant bacterial strains. Examples include tuberculosis, where 40% of cases are resistant to at least one antibiotic, and MRSA (methicillin-resistant Staphylococcus aureus). The continuous development of drug-resistant bacteria raises concerns about the future effectiveness of antibiotics.