Why do diseases exist?
A disease may be caused by external factors such as pathogens or by internal dysfunctions.
For example, internal dysfunctions of the immune system can produce a variety of different diseases, including various forms of immunodeficiency, hypersensitivity, allergies and autoimmune disorders.
How do diseases spread from animals to humans?
In direct zoonosis the disease is directly transmitted from other animals to humans through media such as air (influenza) or through bites and saliva (rabies). In contrast, transmission can also occur via an intermediate species (referred to as a vector), which carry the disease pathogen without getting sick.
How many viruses are there?
There are millions of different types of viruses, although only about 5,000 types have been described in detail. As of September 2015, the NCBI Virus genome database has more than 75,000 complete genome sequences, but there are doubtlessly many more to be discovered.
What diseases started in America?
Numerous diseases were brought to the Americas, including smallpox, bubonic plague, chickenpox, cholera, the common cold, diphtheria, influenza, malaria, measles, scarlet fever, sexually transmitted diseases, typhoid, typhus, tuberculosis, and pertussis.