 Infectious diseases are caused by
pathogenic microorganisms, such as
bacteria, viruses, parasites or fungi; the
diseases can be spread, directly or
indirectly, from one person to another.
How Infections Occur
Droplet Contact
Also known as the respiratory route, it is a typical
mode of transmission among many infectious
agents. If an infected person coughs or sneezes on
another person the microorganisms, suspended in
warm, moist droplets, may enter the body through
the nose, mouth or eye surfaces. Diseases that are
commonly spread by coughing or sneezing include
(at least):
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Bacterial Meningitis
Chickenpox
Common cold
Influenza
Mumps
Strep throat
Tuberculosis
Measles
Rubella
Whooping cough
Direct contact is rare in this route, for humans at least.
More common are the indirect routes; foodstuffs or water
become contaminated (by people not washing their hands
before preparing food, or untreated sewage being
released into a drinking water supply) and the people who
eat and drink them become infected. In developing
countries most sewage is discharged into the
environment or on cropland as of 2006; even in developed
countries there are periodic system failures resulting in a
sanitary sewer overflow. This is the typical mode of
transmission for the infectious agents of (at least):
 Cholera
 Hepatitis A
 Polio
 Rotavirus
 Salmonella
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This refers to any disease that can be caught during sexual activity with
another person, including vaginal or anal sex or (less commonly) through oral
sex. Transmission is either directly between surfaces in contact during
intercourse (the usual route for bacterial infections and those infections causing
sores) or from secretions (semen or the fluid secreted by the excited female)
which carry infectious agents that get into the partner's blood stream through
tiny tears in the penis, vagina or rectum (this is a more usual route for viruses).
 Some diseases transmissible by the sexual route include (at least):
 HIV/AIDS
 Chlamydia
 Genital warts
 Gonorrhea
 Hepatitis B
 Syphilis
 Herpes
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Diseases that can be transmitted by direct contact are called
contagious . These diseases can also be transmitted by sharing a
towel (where the towel is rubbed vigorously on both bodies) or
items of clothing in close contact with the body (socks, for
example) if they are not washed thoroughly between uses. For
this reason, contagious diseases often break out in schools,
where towels are shared and personal items of clothing
accidentally swapped in the changing rooms.
 Some diseases that are transmissible by direct contact
include:
 Athlete's foot
 Impetigo
 Syphilis (on rare occasions, if an uninfected person touches a
chancre)
 Warts
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 A vector is an organism that does
not cause disease itself but that
transmits infection by conveying
pathogens from one host to
another.
 Examples include mosquitoes,
deer ticks, animal bites, etc.
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Is It Contagious?
Non-infectious diseases (also called Noncommunicable diseases ) are those diseases that are
not caused by a pathogen and cannot be shared from
one person to another.
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Non-infectious diseases may be caused by either the
environment, nutritional deficiencies, lifestyle choices,
or genetic inheritances. Unlike infectious diseases, noninfectious diseases are not communicable or
contagious, although some kinds can be passed down
genetically to the children of a carrier.
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 Historically, infectious diseases were the main
cause of death in the world and, indeed, in
some developing regions this may still be the
case. With the development of antibiotics and
vaccination programs, infectious disease is no
longer the leading cause of death in the western
world. Non-infectious disease is now
responsible for the leading causes of death in
both developed and some developing countries.
 Causes of non-infectious disease are
categorized into genetically inherited
diseases and environmental diseases.
Non-infectious diseases that are not
genetic disorders are environmental
diseases, although many diseases are
affected by both genetic and
environmental factors.
 Genetic disorders are caused by errors in
genetic information that produce
diseases in the affected people. These
errors may include:
 A change in the chromosome numbers,
such as Down syndrome
 A defect in a single gene caused by
mutation, such as Cystic Fibrosis.
Environmental disease is a very broad category. It includes
avoidable and unavoidable conditions caused by external
factors, such as sunlight, food, pollution, and lifestyle choices.
The diseases of affluence are non-infectious diseases with
environmental causes. Examples include:
 Many types of cardiovascular disease
 Chronic obstructive pulmonary disease caused by smoking
tobacco
 Type 2 Diabetes
 Malnutrition caused by too little food, or eating the wrong
kinds of food (e.g. scurvy from lack of Vitamin C)
 Skin cancer caused by radiation from the sun
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Cancer is a non-infectious disease that affects
millions of people. The National Cancer
Institute reports the most common cancer type
as nonmelanoma skin cancer, with about one
million cases a year. Lung cancer is the secondmost common, with about 219,440 new cases
and 159,390 deaths. Other common cancers
include bladder, breast, leukemia, pancreatic,
prostate, thyroid, colorectal, endometrial and
kidney cancer.
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 Mayo Clinic reports heart disease, a non-
infectious disease, as the number-one killer in
the United States. Heart or cardiovascular
disease refers to a variety of conditions that
involve the circulatory system. Heart attacks,
strokes, chest pain, tachycardia, high
cholesterol, heart valve disease, high blood
pressure, palpitations and congenital heart
disease are some of the non-infectious heart
diseases.
The National Kidney Foundation reports
chronic kidney disease (CKD) is a non-infectious
disease that affects about 26 million Americans.
CKD results when the kidneys are no longer
able to remove wastes from the body. Diseases
such as diabetes, high blood pressure, lupus,
kidney stones and polycystic kidney disease
may hinder the kidneys' ability to process and
remove wastes.
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 Mental disease covers a broad range of
non-infectious diseases that affects people
of all ages. Autism, panic attacks,
narcolepsy, manic-depressive disorder,
depression, bulimia nervosa, bipolar
disorder and Asperger's syndrome are
examples of non-infectious mental
diseases.
Non-infectious bone diseases include
osteoporosis, Paget's disease and osteogenesis
imperfecta. Osteoporosis indicates a low bone
mass, fragile bones, and high risk of bone
fractures. Paget's disease presents as bones that
grow larger and weaker than normal bones.
Osteogenesis imperfecta (OI) is a non-infectious
genetic disorder. OI symptoms include bones
that break easily.
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Non-infectious muscle and joint diseases include
fibromyalgia, rheumatoid arthritis and gout.
Fibromyalgia is a chronic non-infectious disease
identified as pain and tender points in soft tissue,
muscles and joints throughout the body. Merck
Manual, an online medical library, reports rheumatoid
arthritis (RA) affects about one percent of the world's
population. RA symptoms range from mild occasional
joint pain to severe and debilitating pain. Gout is a
non-infectious disease caused by high levels of uric
acid in the joints and manifests as sudden severe joint
pain, possibly accompanied by fever and feeling ill.
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When carbohydrates are eaten, they get
converted to glucose which is blood sugar. To
stabilize blood sugar levels, the pancreas
releases insulin. In people that have diabetes, a
noninfectious disease, the pancreas does not
release insulin properly or at all in the most
severe cases. This can cause kidney failure,
blindness, nerve damage and heart problems.
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