Infectious diseases are those diseases that can spread from one person to another through various environmental factors such as air, water, and direct contact and through certain medical procedure such as blood transfusion. Many microorganisms such as bacteria, virus and parasites are known as causative agents of many infectious diseases that can spread from one part of the world to another due to increased human movement. Many diseases that were known to occur among a small population has now spread globally since infected individuals could travel from one part of the globe to another in matter of hours carrying the infectious agent with himself and affecting many people in the process. Spread of bird flu is a classic example of rapid spread of infectious diseases all over the world. Spread of HIV in the early 80s also shows how HIV + individual took the disease out of Africa and spreading it all over the world. Growing gay population and the practice of having multiple partners only added to the trouble.
Climatic changes are one of the leading causes of giving birth to new and novel diseases that were not there previously. High rainfall leads to stagnant water that attracts many insects including mosquitoes, these parasites carry many disease, causing microorganisms that rapidly spread through mosquito bite. All it takes is one bite from these dangerous parasites and the infected human carries the disease with him and affects many people he /she come across. And the matter is that these disease-causing viruses are spreading at a fast race like West Nile virus is too rapid to contain. Rift valley fever and Hantavirus pulmonary syndrome are few examples of such diseases.
Every event that occurs in the environment has an impact on the human health. Forests are covered with thick plantation that harbors ticks which are known to cause Lyme disease, under such situation plantation of new forest areas can invite these ticks, and rapid urbanization too can have rapid impact on the climatic condition leading to outbreak of various diseases.
Urban set ups can support only a limited population, rapid urbanization has led to cramped spaces where many people are forced to live in order to sustain themselves. Too many people are compelled to sustain with limited resources, utility lines for water and electricity are overburdened due to population pressure. Such unhygienic living condition causes infectious diseases such as cholera, which is water-borne disease, and dengue that is spread by mosquitoes. As living conditions change so does the diseases, people often suffer from persistent conditions such as allergies and asthma, which are not considered as life threatening but many life threatening diseases are lurking in the darkness that threaten our very existence.