What are the characteristics of viruses?

Viruses are obligate intracellular parasites of nucleic acid and protein. Their RNA or DNA genome is covered with a protein coat. They are among the smallest infectious agents known. They range in size from the tiny polio virus (2 nm diameter) to the large pox virus, which is 400 nm in diameter.

Viruses are also looked upon as highly evolved mobile genetic elements that have cellular origin. Viruses exhibit typical morphological and physical characters including a great variety in size and shape, chemical composition, structure of the genome and mode of replication.

Viruses lack most of the enzymes necessary for the metabolism and synthesis of complex molecules and so can replicate only inside a living cell.

The strandedness of genome of a virus is a characteristic feature. The viral genome may consist of one type of nucleic acid either DNA or RNA which may be single stranded or double stranded. Viruses make us of the ribosomes of their host cells; this defined as absolute parasitism.
What are the characteristics of viruses?

Klinefelter syndrome

Klinefelter syndrome affects n1:660 men, making it the most common sex=chromosome disorder in man, and is a common cause of infertility. The syndrome was named after Harry F. Klinefelter.

It is a chromosomal disorder (47XXY and variants, e.g. 46XY/47XXY mosaicism), i.e. a male with an extra X chromosome. There is both a loss of Leydig cells and seminiferous tubular dysgenesis.

 The presence of Y chromosome explains the male gonadal differentiation with subsequent testosterone and AMH production and the therefore the individual are phenotypically male.

Patients usually present in adolescence with poor sexual development, small or undescended testes, gynaecomastia or infertility.

Teenagers and young men with Klinefelter’s syndrome have a variety of problems, the most severe of which are a tendency toward the body shape of a woman, lack of muscle mass, abnormal breast development, behavioral problems and is some cases, reduced intelligence.
 Klinefelter syndrome

Crohn's disease

Crohn's disease is an intestinal disease characterized by chronic intestinal inflammation. It can affect any area of the gastrointestinal tract from the mouth to the anus.

Crohn’s disease involves the full thickness of the bowel wall. Crohn’s disease can be complicated by fistulas and abscesses.

The most common symptoms of Crohn’s disease are abdominal pain, often in the lower right area, and severe and persistent diarrhea. Rectal bleeding, weight loss, fatigue and fever may also occur. Bleeding may be serious and persistent bleeding to anemia.

Children with Crohn’s disease may suffer delayed development and stunted growth.

Although the true cause of Crohn’s disease still remains unclear, various factors are assumed to play an underlying role in the development of this disease. Among these genetic, immunologic, microbial and dietary factors may have implications.

Environmental factors seem to play an important role in the manifestation, course and prognosis of Crohn’s disease. Epidemiological studies suggest that the prevalence of Crohn’s disease is higher in industrialized countries.
Crohn's disease

Erdheim–Chester disease

Erdheim–Chester disease is a rare non-Langerhans cell histiocytic disorder of unknown cause characterized by heterogeneous systemic manifestation that usually affects adults. The definition of a histiocyte has expanded over time form it original conception as a macrophages to now include both macrophagic and monocytic cells involved in phagocytosis, antigen presentation, and other functions in the immune response.

It was first described as ‘lipid granulomatose’ by Jacob Erdheim’s student William Chester in 1930.

It is microscopically characterized by lipid-laden foamy macrophages, chronic inflammatory cells, and varying degrees of fibrosis.

Extraskeletal manifestations are present in around one half of cases, and the sites of involvement also include the skin, orbit, hypothalamus/pituitary and retroperitoneum: these sites may be more often involved than the lung.

Erdheim–Chester disease is believed to be non-inherited disorder of middle-aged patients with a slight male preponderance. Report published in 2012, there is being a strong male predominance in this disease, with 73% of patients being male and only 27% female.
Erdheim–Chester disease

Xeroderma pigmentosum

Xeroderma pigmentosum is a group of disorders characterized by hypersensitivity of the skin to sunlight, which results in atrophy of the exposed skin and development of pigmentation, telangiectasia, keratoses and cutaneous malignancy in childhood.

These changes result from defective DNA repair of damage done by ultraviolet radiation. When exposed to sunlight the UV radiation induces thymine dimmers and other damage in the DNA of human cells.

Normal individual have several mechanisms for repairing damaged DNA. These mechanisms are absent to various degrees in patients affected with Xeroderma pigmentosum.

Xeroderma pigmentosum was first named by Hebra and Kaposi in 1874 and the association of Xeroderma pigmentosum with neurological dysfunction by de Sanctis and Cacchione in 1932.

Clinical hallmarks of Xeroderma pigmentosum
*Severe photosensitivity
*Poikiloderma
*Dryness (xerosis)
*Premature skin aging
*Malignant tumors (squamos cell cancers, basal cell cancers and melanoma), most often on face, head and neck

Xeroderma pigmentosum is one of the few diseases that can cause poikiloderma at an early age. Poikiloderma is characterized by erythema, hyper and hypopigmentation as well as scaring and telangiectasias.

Xeroderma pigmentosum occurs in all races and affects both sexes equally.
Xeroderma pigmentosum