Infectious diseases have one thing in common-they're all problematic and difficult to treat. The sunset of such diseases can be rapid and unpredictable. However, they also have a curious character: The almost assured survival dichotomy of the patient. The infected person will either die a slow, painful death from the disease or recover completely, forever immune to re-infection. Some of them arise as a result from some type of neglect in our own lifestyles (poor sanitation, infrequent hygiene, etc.). Thankfully, the most common methods of transmission for these diseases are well known and are preventable.
The human immune system has evolved into an intricate mix of organisms, effectively protecting the body from infections and diseases that would have otherwise killed humans thousands of years ago. Influenza, smallpox, tuberculosis and malaria are some of the diseases that routinely prevented people from various centuries ago, but due to technological advances and new medical discoveries, these diseases are either well-vaccinated against the point where they are very rare or eradicatedly. That being said, infections change dynamically; that is, as the human immune system has changed, so have infectious agents. Furthermore, the diseases that cause often arise as a result of something else happens to allow them to flourish. This is how opportunistic infections enter the body after HIV infection.
There are three broad categories of infectious diseases. Established ones are those who have been with us for quite some time and have a predictable mortality and morbidity rate; various respiratory diseases, types of malaria and tuberculosis would fall into this category. Newly emerging infectious diseases are those who are just now appearing in humans for the first time, such as HIV / AIDS. Reemerging infectious diseases are those that "keep coming back" in one way or another, such as influenza.
Respiratory infections kill over 4 million people a year according to current estimates; in fact, over one-quarter of all deaths are caused due to infectious diseases. This is a disabling statistical for many who treat and research these diseases. Vaccines against certain diseases, such as polio and measles, have been in place through the world for quite some time, and as a result the number of new cases has declined. Treating infectious diseases such as HIV has historically proved to be problematic because HIV can hide within the body during treatment. Bacteria can become resistant to antibiotics, and viruses mutate into different forms that render treatment against them ineffective. As a society, we must realize that the challenges of dealing with infectious diseases will never go away. As they continue to evolve and reemerge, the perpetual fight to eradicate them must continue as well.