Quite simply, infectious diseases are illnesses and disorders that are caused by biological organisms ranging from parasites to fungi to viruses. Some can be helpful to the human body; many are harmful. The method of transmission of infectious diseases varies greatly from one to the next. Some can be transmitted from one species to the next, such as through an insect bite. Others are communicated via water droplets in the air. Still others are contained in bodily fluids and are transmitted via exposure. The diversity in the symptoms and indicators of infection are equally diverse; some are very mild and often go unnoticed or are easily remedied by rest, while others are life-threatening and require hospitalization or lifelong medical treatment. Generally, proper handwashing and sanitation practices can help stem the spread of some, but not all, infectious diseases, as the organisms responsible for the infection can be eliminated.
While many infectious diseases are not fatal, globally, approximately half of all deaths – numbering close to five million – related to these ailments are caused by HIV/AIDS, malaria, or tuberculosis. The most common infectious disease in the world, though, is Hepatitis B, impacting more than 2 billion people, or more than 25% of the population of the earth. Those carrying the chronic form of the disease number approximately 350 million. Hepatitis B, also one of the most common infectious diseases, is a chronic condition in 180 million people around the world; between 3 and 4 million new cases emerge each year. Malaria occurs mainly in tropical and subtropical parts of the world, but impacts more than half a billion people each year, many of whom are children; it proves fatal to between 1 and 3 million annually.
Dengue is an infectious disease transmitted to 50 million people each year by a very specific species of mosquito; while the symptoms are not pleasant, it rarely proves fatal. Finally, tuberculosis, caused by a bacteria in the lungs, manifests in nearly 9 million new patients each year; nearly 33% of the global population are carriers of a dormant form of the disease, infected but not able to transmit it. The vast majority of deaths related to tuberculosis occur in countries where education about the disease is altogether lacking, but the overall death rate is, and has been, declining. Together, these five infectious diseases represent the most common, although not the deadliest, biologically transmitted ailments in global society.