This is a brief article about some common infections that may be transmitted via direct or indirect human contact. It includes four categories: viral, bacterial, fungal, and protozoan infections.

1. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 1
HUMAN CONTACT DISEASES
“Human contact diseases are caused by microorganisms that spread via person-to-person
contact or indirect contact with contaminated objects.”
KINDS OF CONTACTS:
 Droplet contact: coughing or sneezing on another person
 Direct physical contact: touching an infected person, including sexual contact
 Indirect physical contact: by touching soil contamination or a contaminated surface
 Fecal-oral transmission: from contaminated food or water sources
CAUSATIVE AGENTS: These diseases are mostly caused by viral, fungal, bacterial, or
protozoan infections. Each causative agent may either directly or indirectly come across its
next unwary victim, and may contaminate several people at once.
DISEASES CAUSED BY VIRUSES
1. COLD SORES: It is a disease in which blisters appear (i.e. around the lip area, etc.), it is
caused by Herpes Simplex Virus, which is a double-stranded DNA virus. Often spread
with direct contact.
 Infection: Blisters develop at the inoculation site and result in tissue destruction,
mostly on gums, lips, and mouth (gingivostomatitis). It heals within a week but the
virus often doesn't leave and instead stays in the trigeminal nerve for maybe a
lifetime. If infection reappears due to stressful stimuli (sunlight, fever, trauma,
chilling, emotional stress, and hormonal changes) then the virus transfers to the
peripheral nerves and may also affect the cornea along with lips and mouth.
 Diagnosis: By ELISA and direct fluorescent antibody screening of tissue. It may also
be made through the recovery of viral nucleic acid by PCR.
2. COMMON COLD: It is a viral infection of the respiratory tract (nose and throat), about
50% of the cases are caused by rhinovirus (rhinos: nose), which are non-enveloped
single-stranded viruses (it has many distinct forms).
 Transmission: It is transmitted via infected individuals (who expel viruses in nasal
secretions), airborne transmission over short distances by way of moisture droplets,
and transmission on contaminated hands or fomites (towels, etc.).
 Infection: Viral invasion of the upper respiratory tract is the basic mechanism in the
pathogenesis of a cold. The virus enters the body’s cells by binding to the adhesion
molecule ICAM-1.
 Clinical manifestations: Nasal stiffness, sneezing, scratchy throat, watery discharge
which becomes yellow after several days. Though fever is absent, but a low-grade
fever (100-102°F) may occur in infants or kids. The diagnosis is done by observing the
symptoms.

2. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 2
3. GENITAL HERPES: It is a sexually transmitted disease, caused by the herpes simplex
virus type 2 (HSV-2).
 Transmission: Mostly caused by sexual contact (Sexually transmitted disease).
 Infection: It begins when the virus is introduced into a break in the skin or mucous
membranes. The virus infects the epithelial cells of the external genitalia, the
urethra, and the cervix. Infection is split into two phases:
 Active Phase: Virus multiplies explosively and new virions are produced from
each infected cell. HSV-2 inhibits its host cell’s metabolism and degrades its DNA.
Cell dies as a result blisters appear on the skin containing infectious liquid. Fever,
headache, burning sensation, and genital soreness are symptoms of this phase.
Blisters may heal after some time but the virus retreats to sacral plexus of spinal
and remain in a latent form.
 Latent Phase: Viral genome incorporates into the host genome. Periodically, the
viruses multiply and migrate down nerve fibers to the skin or mucous
membranes that the nerve supplies, where they produce new blisters. Activation
may be due to sunlight, sexual activity, illness accompanied by fever, hormones,
or stress.
 Cure: No special cure, but some antiviral agents are effective against blisters.
4. LEUKEMIA: It is a tumor of leukocytes and blood-forming tissues (bone marrow), it can
be caused by two retroviruses: (1) Human T-cell lymphotropic virus I (HTLV-I) and (2)
HTLV-II (there are other causes as well).
 Transmission: It is transmitted via sharing needles among drug addicts, sexual
contact, across the placenta, breastfeeding, mosquito bite, etc.
 Infection: Once a cell is infected, the RNA genome is converted by reverse
transcriptase to DNA and integrates into the host's chromosome.
 Hairy Cell leukemia: This leukemia is a chronic, progressive lymphoproliferative
disease. It is caused by HTLV-II. It originates in a stage of β-cell development. The
bone marrow, spleen, and liver become infiltrated with malignant cells, lowering the
person's immunity.
DISEASES CAUSED BY BACTERIA
1. INCLUSION CONJUNCTIVITIS: It is a bacterial infection of the conjunctiva caused by
Chlamydia trachomatis.
 Transmission: Chlamydia is acquired during passage through an infected birth canal.
In adults, it is acquired by contact with infective genital tract discharges.
 Infection: It is characterized by mucous discharge from the eye, inflamed and
swollen conjunctiva, and presence of large inclusion bodies in the host cell

3. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 3
cytoplasm. The disease appears 7 to 12 days after birth. If the Chlamydia colonizes
an infant’s nasopharynx and trachea bronchial tree, pneumonia may result.
 Cure: Recovery usually occurs spontaneously over several weeks or months.
Erythromycin eye drops prevent ophthalmia (inflammation of the eye). Therapy
involves Tetracycline, Erythromycin, Sulfonamides, etc.
2. BACTERIAL VAGINOSIS: It is a polymicrobial disease of the female reproductive canal;
it is caused by Gardnerella vaginalis, Mobiluncus spp. and various anaerobic bacteria.
 Transmission: It is a sexually transmitted disease.
 Infection: It is a mild disease. It can cause various adverse outcomes of pregnancy. It
may also cause pelvic inflammatory disease: it is characterized by a copious, frothy,
fishy-smelling discharge with varying degrees of pain or itching.
 Diagnosis: Diagnosis is based on fishy odor from vaginal secretion, microscopic
observation of clue cells in the discharge (Clue cells are sloughed-off vaginal
epithelial cells covered with bacteria, mostly G. vaginalis).
 Cure: Treatment for bacterial vaginosis is with metronidazole, a drug that kills
anaerobic streptococci and the Mobiluncus spp.
3. CHANCROID: It is a sexually transmitted disease that causes sores on the genitalia; it is
caused by a gram-negative bacillus Haemophilus ducreyi.
 Transmission: It is a sexually transmitted disease.
 Infection: It enters the skin through broken epithelium. After incubation of 4-7 days,
papular lesions develop within the epithelium, causing swelling and white blood cell
infiltration. A pustule forms and ruptures, causing a painful ulcer.
 Diagnosis: Isolating H. ducreyi from the ulcers.
 Cure: It can be cured via chemotherapeutical agents, i.e. azithromycin or ceftriaxone.
4. TETANUS: It is caused by Clostridium tetani, an anaerobic, gram-positive, endospore-
forming rod.
 Transmission: Associated with skin wounds. Any break in the skin can allow C. tetani
spores to enter (if the environment is exposed to its spores, i.e. horse dung).
 Infection: If oxygen tension is low, spores germinate and release the neurotoxin
tetanospasmin, which results in uncontrolled stimulation of skeletal muscles (spastic
paralysis). A second toxin, tetanolysin, is a hemolysin that aids in tissue destruction.
 Course of Disease: It causes tension or cramping and twisting in skeletal muscles
surrounding the wound and tightness of the jaw muscles. With more advanced
disease, there is trismus/lockjaw, an inability to open the mouth. Facial muscles may
go into spasms. The spasms or contractions of the trunk and extremity muscles may
be very severe. Death usually results from spasms of the diaphragm and intercostal
respiratory muscles.

4. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 4
 Prevention: By immediately getting a tetanus booster shot (passive immunity).
Proper care of wounds contaminated with soil.
5. TRACHOMA: It is caused by Chlamydia trachomatis (different variants than that of
conjunctivitis). It is one of the oldest known infectious diseases of humans and is the
greatest single cause of blindness throughout the world.
 Transmission: It is transmitted by contact with infected objects (i.e. as soap and
towels), by hand-to-hand contact, from an infected eye to an uninfected eye, by flies
(bite).
 Infection: It begins abruptly with an inflamed conjunctiva, leading to an
inflammatory cell exudate (fluid that seeps out) and necrotic eyelash follicles. It
heals spontaneously. If reinfection occurs, scarring of the conjunctiva can occur, if
scar tissue accumulates over the cornea, blindness results.
 Treatment: Erythromycin eye drops prevent ophthalmia (inflammation of the eye).
Therapy involves Tetracycline, Erythromycin, Sulfonamides, etc.
DISEASES CAUSED BY FUNGI
1. SUPERFICIAL MYCOSES: Several fungal infections are called superficial because they
are limited to the skin surface. They include:
 Black piedra (fungal infection of hair shafts – black color) is caused by Piedraia hortae.
 White piedra (fungal infection of hair shafts – white color) is caused by the yeast
Trichosporon beigelii.
 Tinea versicolor (fungal infection of the skin) is caused by the yeast Malassezia furfur.
 Infection: Black piedra forms hard, black nodules on the hairs of the scalp. White
piedra forms light-colored nodules on the beard and mustache. Tinea versicolor
forms brownish-red scales on the skin of the trunk, neck, face, and arms.
 Treatment: Removal of the skin scales with a cleansing agent. Removal of the
infected hair. Usage of antifungal agents (surficial).
2. SUBCUTANEOUS MYCOSES: These diseases are caused by normal saprophytic
inhabitants of soil and decaying vegetation.
 Chromoblastomycosis is caused by the black molds Phialophora verrucosa and
Fonsecaea pedrosoi.
 Maduromycosis is caused by Madurella mycetomatis.
 Sporotrichosis is caused by the dimorphic fungus Sporothrix schenckii.
 Transmission: Most infections involve barefooted agricultural workers when they
are wounded on the infected surface.

5. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 5
 Infection: Because they are unable to penetrate the skin they are called
subcutaneous. The disease develops slowly, often over the years. During this time
the fungi produce a nodule that eventually ulcerates and the pathogens spread along
lymphatic channels, producing more subcutaneous nodules.
a. In Chromoblastomycosis, most infections involve the legs and feet.
b. In Maduromycosis, the fungus destroys subcutaneous tissue and produces
serious deformities (also known as fungal tumor).
c. Sporotrichosis is caused by a puncture wound from a thorn or splinter
contaminated with the fungus. After an incubation period of 1 to 12 weeks, a
small, red papule arises and begins to ulcerate. New lesions appear along lymph
channels and can remain localized or spread throughout the body.
 Cure: The treatment involves the usage of antifungal agents or the surgical removal
of the fungus.
DISEASES CAUSED BY PROTISTS
1. TOXOPLASMOSIS: It is caused by the protist Toxoplasma gondii. It has been found in
nearly all animals and most birds. Cats are the definitive host and are required for
completion of the sexual cycle.
 Transmission: Animals shed oocysts in the feces; the oocysts enter another host by
way of the nose or mouth; and the parasites colonize the intestine. Toxoplasmosis
also can be transmitted by ingestion of raw or undercooked meat, congenital (by
birth) transfer, blood transfusion, tissue transplant, etc.
 Infection: Most cases of toxoplasmosis are asymptomatic. In immune-incompetent
or immune-suppressed individuals, it frequently results in fatal disseminated (that
spreads) disease with a heavy cerebral involvement. Acute toxoplasmosis is usually
accompanied by lymph node swelling with reticular cell hyperplasia (enlargement of
reticular cells in the lymph nodes). Pulmonary necrosis, myocarditis, and hepatitis
are caused by tissue necrosis. Toxoplasma in immune-compromised individuals, such
as AIDS or transplant patients, can produce a unique encephalitis (inflammation of
the brain).
 Treatment: It can be treated with a combination of pyrimethamine and sulfadiazine.
 Prevention and Control: It can be prevented by minimizing exposure by avoiding
eating raw meat and eggs, washing hands after working in the soil, cleaning cat litter
boxes daily, keeping household cats indoors if possible, and feeding them
commercial food.
2. TRICHOMONIASIS: It is a sexually transmitted disease caused by a protozoan flagellate
called Trichomonas vaginalis.
 Transmission: It is a sexually transmitted disease.

6. SYED MUHAMMAD KHAN (BS HONS. ZOOLOGY)
pg. 6
 Infection: In response to the parasite, the body accumulates leukocytes at the site of
the infection. In females, this usually results in a profuse, purulent (pus) vaginal
discharge that is yellowish to light cream in color, and has a bad odor. The discharge
is accompanied by itching. Males are generally asymptomatic because of the
trichomonacidal action of prostatic secretions, but burning sensation occurs during
urination.
 Diagnosis: In females, it can be diagnosed by microscopic examination of the
discharge and identification of the protozoan. Infected males have the protozoan in
their semen or urine.
 Treatment: By administration of metronidazole, i.e. Flagyl (an antimicrobial agent).
AVOIDING HUMAN CONTACT DISEASES: To avoid these, and many other similar
diseases, one must avoid human contact wherever possible, especially when one doubts
that a person may be an infection carrier. Social distancing protocols, proper sanitation,
perfect hygiene regulations, and adopting a clean and healthy lifestyle will ensure that you
don’t come across such diseases.