Influenza Viruses (Orthomyxoviridae and Paramyxoviridae)

Structure
Viruses are spherical, containing negative stranded RNA; outer membrane contains hemagglutinin activity (HA) and neuraminidase activity (NA) glycoproteins that are anchored to the inner lipid bilayer by M-proteins.

Hemagglutinin
Attachment to host cell sialic acid receptors, which are found on the surface of red blood cells, and upper respiratory tract cells. HA is needed for absorption of the viral genome.

Neuraminidase
Cleaves neuraminic acid of the mucin upper respiratory barrier to expose sialic acid receptors; also cleaves sialic acid receptor to avoid attachment of budding viruses, which will escape to infect new cells. 

In paramyxoviruses, HA and NA are part of the same glycoprotein spike. They also contain fusion protein (not found in orthomyxoviruses) that causes the infected cells to fuse into giant multinucleated cells.

Orthomyxoviridae
Orthomyxoviruses are influenza viruses, of which there are three types. Type A infects humans, other mammals, and birds, whereas type B and C infect only humans. Type A causes the most severe disease, with Type B and C in decreasing severity.

Paramyxoviridae (colds/flu in adults, pneumonia in children, measles, mumps)
Paramyxoviruses include parainfluenza viruses, respiratory syncytial virus (RSV), metapneumovirus, mumps virus, and measles virus.

Metapneumovirus causes upper and lower respiratory tract infections primarily in young children or elderly. Parainfluenza and RSV cause upper respiratory infections that have cold-like symptoms in adults, but produce influenza-like sickness from lower respiratory tract infections (bronchiolitis, viral pneumonia, croup) in children, elderly, and immunocompromised patients.