proteins that the host cell’s RNA transcribe as
instructions to replicate the virus’s DNA, which
the cell does. DNA contains the instructions for
the cell’s functions; RNA forms the messenger pro-
teins that carry out the directives of the DNA.
Eventually the virus’s copies of DNA crowd out
the cell’s copies of DNA, and the cell becomes the
agent of the virus. The cell either divides or rup-
tures, spreading the virus. RNA viruses achieve a
similar result by causing the host cell to replicate
their RNA, which then replaces the cell’s RNA.
Retroviruses are RNA viruses that contain the
enzyme reverse transcriptase, which allows RNA
to instruct DNA (the reverse of normal).
Viruses are highly adaptable and have numer-
ous mechanisms to hide from the IMMUNE SYSTEM,
allowing them to become well established infec-
tions before the immune system detects their pres-
ence. Once the immune system does detect a
virus, it develops antibodies that protect against
subsequent infection by the same virus. Many
common viruses—such as those that are responsi-
ble for COLDS (rhinoviruses), GASTROENTERITIS
(enteroviruses), and the flu (INFLUENZAviruses)—
frequently alter their structures, evolving into dif-
ferent strains that can cause the same illnesses.
Some viruses, such as human T-lymphotropic
virus (HTLV) and HUMAN PAPILLOMAVIRUS (HPV),
cause cancer (oncoviruses). The human immuno-
deficiency virus (HIV) is a retrovirus that attacks
the immune system, causing AIDS (acquired
immunodeficiency syndrome).
See also ANTIBODY; ANTIBODY-MEDIATED IMMUNITY;
CELL-MEDIATED IMMUNITY; HIV/AIDS.waterborne illnesses Diseases that result from
pathogens transmitted by drinking or otherwise
consuming contaminated water. Heavy metals and362 Infectious Diseases
Virus or Viral Family Genetic Configuration Illness
human papillomavirus (HPV) DNA genital WARTS, CERVICAL CANCER, vaginal cancer
human parainfluenza viruses RNA acute upper respiratory disease, CROUP, bronchiolitis, BRONCHITIS,
pneumonia
INFLUENZAviruses RNA influenza (flu)
MEASLESvirus RNA measles
MUMPSvirus RNA mumps
Norwalk-like viruses RNA acute gastroenteritis
poliovirus RNA POLIOMYELITIS
RABIESvirus RNA rabies
respiratory syncytial virus RNA bronchiolitis, pneumonia, acute upper respiratory disease
rhinoviruses RNA COLDS
RUBELLAvirus RNA rubella (German or three-day measles)
varicella-zoster DNA CHICKENPOX, HERPES ZOSTER(shingles)
variola DNA SMALLPOX