Infectious diseases

INFECTION AND INFECTIOUS DISEASE 

INFECTIOUS AGENTS 

The world is full of microorganism ,the vast majority of which are harmless to man and many of which are essential to life .Some of these organism live on or within human hosts: most of these form part of our normal flora and are benign passenger or symbiotes.A minority ,however ,ar pathogenic ,causing illness or even death to their host .It is thses viruses ,bacteria , protozoa ,and worms which are responsible for the infectious disease.

Infection remains the main cause of morbidity and mortality in man ,particularly in underdeveloped areas where it is associated with poverty and overcrowding .In the developed world increasing prosperity ,universal immunization and antibiotics have reduced the prevalence of infectious disease .However ,antibiotic -resistant strains of bacteria as well as viruses and ' new ' diseases such as human immunodeficiency virus ( HIV ) infection and variant Creutzfeldt-jakob disease ( vCJD) have emerged .Increased global mobility has aided the spread of infectious disease and allowed previously localized pathogens to establish themselves world-wide .Deteriorating social conditions in the inner city areas of our major conurbations have facilitated the resurgence of tuberculosis and other infection.Changes in farming and food-processing methods have controlled to an increase in the incidence of food and water-borne disease.

In the developing world successes such as the eradication of smallpox have been balance or outweighed by the new plagues.Infectious cause nearly 25% of all human deaths .Two billion people -one third of the world's population-are infected with tuberculosis ,250-300 million people catch malaria every year ,and 200 million are  infected with schistosomiasis .Infections are often multiple and there is synergy both between different infections ,and between infection and other factors such as malnutrition.Many of the infectious diseases affecting developing countries are preventable or treatable ,but continue to thrive owing to lack of money and political will.

INFECTIOUS AGENTS

The causative agents of infectious diseases can be divided into four groups.

Prions  are the most recently-recognized and the simplest infectious agents ,consisting of a single protein molecule.They contain no nucleic acid and therefore no genetic information : their ability to propagate within a host relies on inducing the conversion of endogenous protein into prion protein.

viruses  contain both protein and nucleic acid and so carry the genetic information for their own reproduction.However ,they lack the apparatus to replicate autonomously ,relying instead on "hijacking " the cellular machinary of the host .They are smalll ( usually less than 200 nanametres in diameter ) and each virus possesses only one species of nucleic acid ( either RNA or DNA ).

Bacteria  are usually though not always ,larger than viruses .Unlike the latter they have both DNA and RNA , with the genome encoded by DNA ,They are enclosed by a cell membrane ,and even bacteria which have adopted an intracellular existence remain enclosed within their own cell wall. Bacteria are capable of fully autonomous reproduction ,and the majority are not  dependent on host cells.

Eukaryotes are the most sophisticated infectious organism ,displaying sub-cellular compartmentalization .Different cellular functions are restricted to specific organelles ,e.g photosynthesis takes place in the choloroplast ,DNA transcription in the nucleus and respiration in the mitochondria ,Eukaryotic pathogens include unicellular protozoa ,fungi ( which can be unicellular or filamentous ) ,and multicellular parasite worms.

Other higher classes ,notably the insects and the arachnids ,also contain species which can parasitize man and cause disease .