Infections with viruses

Children and elderly people are at increased risk, especially in developing countries. The most important etiologic agents of severe lower respiratory illness are bacteria such as Streptococcus pneumoniae and Haemophilus influenzae and viruses such as respiratory syncytial virus RSV and influenza virus.

Efficacious vaccines are available against the two bacteria and the influenza virus. Viruses are much more important in mild upper and middle respiratory tract infections and in bronchiolitis in children, whereas bacteria are the main cause of pneumonia, especially in adults.

Clinical syndromes overlap considerably, and there is increasing evidence of bacterial-viral co-infections and of bacterial pneumonia being secondary to viral respiratory tract infection. Disease is mostly limited to the upper airways and is self-limiting, but a small percentage can progress to lower respiratory tract infections LRTIs as bronchiolitis and pneumonia.

Children and elderly people are at increased risk, especially in low- and middle-income countries LMICs. Childhood pneumonia is the leading cause of mortality in children under 5 and has an incidence of 0. This translates into million new episodes annually worldwide, of which million are in LMICs. On the other side of the age spectrum, pneumonia is also a major cause of morbidity and mortality in older people, with an annual incidence for non-institutionalized patients at 25 to 44 per , up to four times that of patients under The most important etiologic agents of severe LRTI are bacteria such as Streptococcus pneumoniae and Haemophilus influenzae and viruses such as respiratory syncytial virus RSV and influenza virus.

Viruses are more important in mild upper and middle respiratory tract infections RTIs and in bronchiolitis in children, whereas bacteria are the main cause of pneumonia, especially in adults.

Despite this, RTIs—because of the difficulty of making a rapid diagnosis of the etiologic agent—are often treated with antibiotics and are responsible for a large part of inappropriate and appropriate use of antibiotics in humans.

Clinical syndromes overlap considerably, and there is increasing evidence of bacterial-viral co-infections and of bacterial pneumonia being secondary to viral RTI. A range of different viruses can cause RTIs; the most important are the ortho- and paramyxoviridae, picornaviridae, coronaviruses, and adenoviruses.

This percentage is higher in children, particularly children under 5, due to the large proportion of cases caused by RSV. In children under 5 with LRTIs, strong causal associations for RSV, influenza viruses, parainfluenza viruses, and metapneumovirus were found in comparison with healthy controls, and less so for adenoviruses, bocaviruses, and coronaviruses.

Orthomyxoviridae are divided into four genera: A—D. Key amino acids in these proteins, especially in HA, are associated with host specificity and transmissibility in humans. Aquatic birds are the natural reservoir of influenza A viruses, harboring all possible subtypes, except H17, H18, N10, and N These were recently identified in bats, suggesting birds are not the only reservoir.

A selection of subtypes has established endemicity among land and water mammals e. Influenza B and C viruses are mainly human pathogens, with rare reports of influenza B virus infection in dogs, cats, swine, and seals. Influenza C rarely causes human infections, and influenza D viruses mostly infect cattle and will not be further discussed. The reservoir of influenza A viruses.

The working hypothesis is that wild aquatic birds are the primordial reservoir of all influenza viruses for avian and mammalian species. Transmission of influenza has been demonstrated between pigs and humans solid lines.

There is extensive evidence for transmission between aquatic birds and other species, including pigs and horses, and indirect transmission to humans through pigs and evidence for direct transmission to humans from chickens. New lineages of influenza A virus emerge every few decades, resulting in global pandemics with varying severity due to the absence of immunity in the human population. New human viruses have emerged through antigenic drift in avian viruses Spanish flu: H1N1, 40— million deaths or through reassortment of gene segments in animal hosts infected with two different viruses antigenic shift; Asian flu: H2N2, 2 million deaths; Hong Kong flu: H3N2, , deaths.

After such an introduction, the new virus becomes the dominant circulating lineage. Two notable exceptions were the reintroduction of H1N1 in the human population in , possibly caused by an escape from a research laboratory, and the introduction in of a novel lineage of H1N1 influenza A virus, most likely from pigs, in North America, causing a relatively mild pandemic with an estimated ,—, deaths and generating massive attention from both the public health communities and the general public and replacing only the H1N1 lineage.

Currently, H1N1pdm09 and H3N2 influenza A virus are co-circulating with influenza B viruses and continue to cause yearly seasonal epidemics worldwide. With no animal reservoirs to provide such new antigens, shift does not occur in influenza B and thus major epidemics do not occur Fig. Timeline of circulating subtypes of influenza A virus from onwards. In , H1N1 was re-introduced and has since co-circulated with H3N2.

In , a new H1N1 lineage pdm09 was introduced, which has replaced the former H1N1 subtype. Sporadic dead-end human infections of animal viruses are known to occur and have caused concern about the pandemic potential of these viruses. Highly pathogenic H5N1 viruses were first detected in birds in in China. Transmission to 18 humans occurred in Hong Kong, 6 of which were fatal. During the next 6 years no human or animal cases were recorded.

In the virus reemerged in China. Since then H5N1 viruses have become panzootic among poultry and wild birds and have caused sporadic infections fatal in humans until , most of whom reported close contact with wild birds or domestic poultry.

Furthermore, a total of cases including deaths of human infection with avian influenza H7N9 have been reported from China, occurring in annual outbreaks between December and February since Human H7N9 infection is also zoonotic, with no reported sustained human-to-human transmission, and mostly associated with exposure to live bird markets.

Interestingly, human cases have been confined to parts of mainland China, and avian infection is limited to domestic poultry with little spillover to migratory birds and wild waterfowl. The case fatality of H7N9 is lower than H5N1, and associations with age and comorbidities are reported.

Despite their worldwide presence for many years and the huge human—animal interface in Asia, no efficient human-to-human transmission episodes have been recorded for either. However, the continuous evolution and reassortment in these viruses e. These six human viruses are the most important causes of LRTIs in children worldwide. RSV is the single most important cause of bronchiolitis and the leading cause of RTIs requiring hospitalization.

Human metapneumovirus hMPV causes similar disease but less frequently, whereas the parainfluenza viruses are associated with bronchiolitis and croup. A recent study estimated that RSV caused 3. Facebook Twitter LinkedIn Syndicate. Minus Related Pages. What CDC Does. To receive weekly email updates about Seasonal Flu, enter your email address: Email Address.

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CDC is not responsible for Section compliance accessibility on other federal or private website. Cancel Continue. Many neurologic viruses are spread through the bite of an infected animal or bug, such as a mosquito or tick. Other viruses, such poliovirus and other enteroviruses, are quite contagious and spread through close contact with someone with the virus.

Contaminated objects can also contribute to the spread of these viruses. Getting plenty of rest, staying hydrated, and taking OTC anti-inflammatories to ease pain or headaches can all help. In some cases, antiviral medication may be prescribed. Polio or severe cases of meningitis or encephalitis may require additional treatment, such as breathing assistance or IV fluids. Practicing good hygiene, avoiding close contact with those who have the virus, and protecting against insect bites can all help to reduce the spread of encephalitis and meningitis.

To reduce the risk of spreading rabies, keep your pets vaccinated and avoid approaching wild animals. There are many viral diseases. Some, such as the common cold or the stomach flu, are minor and go away on their own within a few days. Others, however, are more serious. Instead, treatment usually focuses on managing symptoms and supporting the immune system with plenty of rest and hydration. Post-viral fatigue is a type of extreme tiredness that sometimes happens after a recent viral infection.

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Health Conditions Discover Plan Connect. Gut ; 63 — Park SH, Rehermann B. Immune responses to HCV and other hepatitis viruses. Immunity ; 40 — PLoS One ; 6 :e Clin Vaccine Immunol ; 19 —9. Immunity ; 29 —9. Immunity to cytomegalovirus in early life. Front Immunol ; 5 Klenerman P, Oxenius A. T cell responses to cytomegalovirus. Nat Rev Immunol ; 16 — Ploegh HL. Viral strategies of immune evasion. Science ; — J Clin Virol ; 41 — Colman R, Blackbourn DJ.

Risk factors in the development of Kaposi's sarcoma. AIDS ; 22 — Nature ; —9. Blood ; — J Immunol ; — J Allergy Clin Immunol ; —9. J Allergy Clin Immunol ; — Science ; —2. Viral interleukin 10 is critical for the induction of B cell growth transformation by Epstein—Barr virus. J Virol ; 83 — J Clin Invest ; — Front Immunol ; 4 A review supporting the hypothesis.

Autoimmun Rev ; 7 — Natural selection and infectious disease in human populations. Nat Rev Genet ; 15 — Am J Trop Med Hyg ; 85 —4. Parasitology ; Suppl :S— Infect Immun ; 76 — Nat Med ; 6 — PLoS One ; 9 :e Prolongation of egg production of Nippostrongylus brasiliensis in mice concurrently infected with Nematospiroides dubius. J Parasitol ; 59 Impairment of primary expulsion of Trichuris muris in mice concurrently infected with Nematospiroides dubius.

Parasitology ; 75 —8. Trichinella spiralis : delayed rejection in mice concurrently infected with Nematospiroides dubius. Exp Parasitol ; 46 — J Infect Dis ; — Infect Immun ; 73 —9. Clin Infect Dis ; 54 — Helmby H.