Some viruses enter their host cells by attaching to cell-surface
carbohydrates. The influenza virus in particular, attaches to
sialic acid residues on the terminai of the oligosaccharides
presentnear the cell-surface glycoproteins and glycolipids. Once
attached, viruses inject their own genetic material and take
over the cell's machinery to produce more viruses. The cell can
undergo a lytic or lysogenic cycle. In the lytic cell, the cell
is taken over, produces viruses with its own machinery and
organelles, and then dies, releasing more viruses. In the
lysogenic cycle, the cell does not die but instead replicates
with viral DNA/RNA in its own genome. A common example of a
lysogenic virus is lambda phage. However, lambda phage can also
enter the lytic cycle. In this manner, some viruses can remain
dormant for years at a time, activating under certain
environmental conditions to begin replication under the lytic
cycle. There are several different types of viruses, including
DNA, RNA, and retroviruses. DNA virus