A primer on human metapneumovirus (hMPV)...

Human metapneumovirus (hMPV) was first described by van den Hoogen et al in 2001.[1] It had been isolated from young children with respiratory tract disease and it was found, by studying old sera, that hMPV had been present in the European community for more than 50 years. The presence of hMPV was next reported in the Australian paediatric population.[2] It is now well known that hMPV is one of many endemic and global human respiratory viruses.

The first reported detection of hMPV outside of the Netherlands was in Queensland,
Australia. I used an in-house conventional reverse-transcriptase polymerase chain reaction to
screen samples suspected of infection by a respiratory  virus, but negative for the "usual suspects". 
Yellow star indicates the positive specimen extract

HMPV infection can result in signs and symptoms that are impossible to differentiate from those due to infection with any other common respiratory virus. An infected individual may remain asymptomatic or show clinical features including rhinorrhoea, cough, shortness of breath, wheeze, vomiting, pharyngitis, chest wheeze with crackles, bilateral lung cellular infiltrates and bronchiolitis.

HMPV genotypes. Aligned in Geneious v8.1.
Neighbor-Joining tree, drawn to scale 
using MEGA v6; 500 bootstraps

This is a member for the Order Mononegavirales,(single strand of negative sense genetic material) Family Paramyxoviridae, Subfamily Pneumonia, genus Metapneumovirus, species Human metapneumovirus.[5] The closest genetic relative remains the avian pneumovirus (APV). HMPV variants exist as two serotypes (reflected by two genetic lineages) called A and B.[3]
Viruses like these have their genetic material, RNA, covered in a protein which, together with other material, is contained within a lipid envelope. The exceptions are the viral proteins which protrude from the envelope. HMPV RNA has been detected in large and small droplets expelled from infected individuals.[4] As with any respiratory virus, containing its spread is difficult once it has become established in health care settings, homes or other close-quarter environments.

Data from Queensland. Figure adapted with permission, 
from Mackay, Ian M., Arden, Katherine E. and Lambert, 
Stephen B. (2009). Epidemiology. In Ronald Eccles and 
Olaf Weber (Ed.), Common Cold 
(pp. 77-106). Basel Switzerland: Birkhäuser Basel. 
doi:10.1007/978-3-7643-9912-2_4

HMPV causes initial disease in a similar age-group to respiratory syncytial virus (RSV; a well known childhood respiratory virus pathogen)- the majority of children are infected with hMPV by five years of age. In addition, the virus can iHmpact severely upon the elderly and the immunocompromised.

HMPV infections peak with a seasonality in temperate climates. The spring and autumn periods tend to show more activity, but virus can circulate all year around, as is the case for all respiratory viruses.

Scale comparison of HMPV, APV and RSV genomes.

References...

1. A newly discovered human pneumovirus isolated from young children with respiratory tract disease
   Bernadette G. van den Hoogen, Jan C. de Jong, Jan Groen, Thijs Kuiken, Ronald de Groot, Ron A.M. Fouchier & Albert D.M.E. Osterhaus
   http://www.nature.com/index.html?file=/nm/journal/v7/n6/abs/nm0601_719.html&dynoptions=doi1020034962
2. Evidence of human metapneumovirus in Australian childrenMichael D Nissen, Ian M Mackay, Stephen J Withers, David J Siebert and Theo P Slootshttps://www.mja.com.au/journal/2002/176/4/evidence-human-metapneumovirus-australian-children
3. Antigenic and genetic variability of human metapneumoviruses.
   van den Hoogen BG, Herfst S, Sprong L, Cane PA, Forleo-Neto E, de Swart RL, Osterhaus AD, Fouchier RA.
   http://www.ncbi.nlm.nih.gov/pubmed/15200856
4. Respiratory virus RNA is detectable in airborne and droplet particles.
   Gralton J1, Tovey ER, McLaws ML, Rawlinson WD.
   http://www.ncbi.nlm.nih.gov/pubmed/23959825
5. http://www.ictvonline.org/virusTaxonomy.asp