More and more people die every year due to infectious diseases all over the world than any other single cause of death. It is estimated by The Center for Disease Control and the World Health Organization that 5 million people die every year from infectious diseases. The number mean can be even higher, but since it is not easy to get exact statistics because most of these deaths take place in developing countries where there is restricted access to sufficient medical care.
Infections are disseminated by microscopic organisms, or germs, as they are commonly known. These include bacteria, viruses, fungi, protozoa, some multicultural parasites and venomous types of rogue proteins called prions. Infections are not only restricted to animals - almost all forms of multicellular life, as well as plants, are susceptible.
Most bacteria are undamaging and even beneficial to humans. The term human flora is used to depict the 500 to 1000 species of bacteria that dwells on and inside us out of which a large number play an important role in preserving good health. Bacteria that are harmful to humans are called pathogenic bacteria and are responsible for diseases such as pneumonia, typhoid fever, syphilis, diphtheria and leprosy.
The most common bacterial disease is tuberculosis, which kills an estimated 2 million people every year. Many of the common diseases like the common cold, flu, cold sores and chicken pox are all caused by viruses. These viruses are also the source of some of the most deadly infectious diseases known like AIDS, Ebola, SARS as well as hepatitis and herpes.
Protozoa are single celled organisms that play a significant role in maintaining the food chain in many biomes, but a few species have induced the deaths of millions of humans worldwide every year in the form of malaria and amoebic dysentery. Certain fungi can find a way around the body’s natural barriers and contaminate it inducing fungal infection. Prions are little understood infectious agents in the form of a misfolded protein that causes neurological diseases like bovine spongiform encephalopathy, popularly known as “mad cow disease,” in cattle and Creutzfeldt-Jakob disease in humans. Both diseases have no cure and are fatal.
The distinction has to be made between the infection itself and the disease resulting from such infection. The infection takes place when an organism enters and multiplies within the host while the infectious disease is indications caused by the unfavorable effects the organism has on the host.
Germs are infinitesimal and are found everywhere on our planet. Consequently, infections can spread through water, through the soil and even through air. Infectious diseases classed as contagious diseases are communicated from one person to another or from one species to another. Transmittable diseases in humans are generally transmitted through kissing and sexual contact, the exchange of bodily fluids, animal and insect bites, airborne inhalation or contact with infected water.
Many microorganisms are localized in remote or isolated habitats. Unavoidably human intervention interrupts their environment making them to find new hosts and take up a new niche in the ecosystem. Wars, overpopulation, modern transportation, clearing woodland for cultivation and to produce new settlements in areas where animal populations and parasites were formerly isolated from humans play a huge role in this. In most of these cases, humans are inadvertent hosts and are not at all adjusted to the new pathogenic species, which result in high mortality rates. The rise and spread of SARS, avian flu, Ebola, West Nile and even the HIV virus that causes AIDS can be assigned to the influence of man. The Center for Disease Control and Prevention has a portal dedicated to discussing emerging diseases. Overusing medications, especially antibiotics have lead to the rise in new breeds of medicine resistant bacteria, popularly called “superbugs”. The New Yorker Magazine has article that takes an in-depth look at the rise of superbugs.
A list of infection and mortality rates of the most prevalent infectious diseases can be found here.