Why is it that in the past there were very few diseases than today?

The ever increasing human population and greater global connectivity today, provides rapid dissemination of infectious diseases from the initial focus. Whereas in previous centuries a disease focus might have died out through failing to establish a chain of transmission, now it has the opportunity to rapidly recruit susceptible hosts on a global stage. To draw an analogy, there is now so much available tinder on the forest floor that the flickering early flame can rapidly be fanned into a forest fire. In support of this thesis, consider the following:

• The human population of Earth took until 1800 to reach one billion; by 2000 it had
  exceeded six billion; and it reached the seven billion mark in 2011;
• In 1800, the time taken to circumnavigate the globe by sailing ship was approximately one year. Today, no two cities served by commercial aircraft are more than 24 hours apart;
• Annually, the world’s airlines carry a total approaching two billion passengers. At any one moment, about half a million people worldwide are flying in commercial aircraft ;
• In 2011, there were 219 million passenger departures/arrivals at British airports;
• In lieu of precise trade data, Billy Karesh of the Wildlife Conservation Organization in New York conservatively estimates that in east and southeast Asia, tens of millions of wild animals are shipped each year regionally and from around the world, for food or use in traditional medicine (Karesh and others 2005).

Wildlife often acts as a reservoir for diseases of domestic animal and humans see figure Figure 1.2 published in an article in Science in 2000 (Daszak et al. 2000).

Most emerging diseases exist within a host and parasite continuum between wildlife, domestic animal, and human populations. Few diseases affect exclusively any one group, and the complex relations between host populations set the scene for disease emergence. Examples of emerging infectious diseases that overlap these categories are canine distemper (domestic animals to wildlife), Lyme disease (wildlife to humans), cat scratch fever (domestic animals to humans) and rabies (all three categories). Arrows denote some of the key factors driving disease emergence (Daszak et al. 2000).

References

Daszak P, Cunningham AA, Hyatt AD. (2000) Emerging infectious diseases of wildlife-threats to biodiversity and human health. Science, 287: 443-449. Available at: http://science.sciencemag.org/content/287/5452/443.full

Gibbs EPJ. (2016): Week One Lecture notes for the course: An Introduction to trans-boundary diseases and their impact on trade and wildlife populations. University of Edinburgh-MSc One Health. Available at: https://www.learn.ed.ac.uk/bbcswebdav/pid-1677759-dt-content-rid-3167481_1/courses/ls_transboundary_diseases_2015/2015%202016/Week%201/Week%201%20Lecture%20Gibbs%20EM%20FINAL%202016copy.pdf