Strictly speaking, penetration occurs when a projectile enters a target without passing through it and "perforation" occurs when the projectile completely passes through the target, but the word "penetration" is commonly used to refer to either.

Penetration into a "semi-infinite" or "massive" target is penetration (in the strict sense of the word) of targets so thick that the level of penetration is not affected by the target's thickness. There is a "transition region" between semi-infinite penetration and perforation, in which the target is not perforated but the projectile, as it nears the back face of the target, meets reduced resistance and is capable of penetrating a greater distance than it would in a semi-infinite target. This effect is variously named the back or rear surface, plate, or face effect and is also present when perforation occurs.

A penetrating projectile may cause the target to break into multiple pieces, spewing from both the front and back of the target, themselves at high velocity. These pieces are collectively referred to as spall. Spall can be generated even if a perforation is not achieved (the projectile fails to pass through the target), generated instead by the shock wave generated by the impact of the projectile.

Bombs designed for great penetration into the earth or for perforation of hardened targets are known as bunker busters.

The smear layer is a layer of microcrystalline and organic particle debris that is found spread on root canal walls after root canal instrumentation. It was first described in 1975 and much research has been performed since then to evaluate its importance in assisting or preventing the penetration of bacteria into the dentinal tubules. More broadly, it is the organic layer found over all hard tooth surfaces.

Cuterebriasis is a parasitic disease affecting rodents, lagomorphs (hares, rabbits, pikas), felines and canines. The etiologic agent is the larval development of bot flies within the "Cuterebra" or "Trypoderma" genera, which occurs obligatorily in rodents and lagomorphs, respectively. Felines and canines serve as accidental hosts, but research suggests only by "Trypoderma" spp. Entrance into the body by first instar larva occurs via mucous membranes of natural orifices or open wounds as opposed to direct dermic penetration.

The symptoms of this disease include:

- Severe pruritus

- Pain

- Inflammation and swelling

- Lesions and ulcerations, with black dots in the center

Left untreated, secondary infections, such as bacteremia, tetanus, and gangrene, can occur.

In all cases, tungiasis by itself only caused morbidity, though secondary infection may lead to mortality. The life cycle section presents the Fortaleza stages from the flea’s developmental perspective. It should be noted that the discussion is specific to symptoms of human infection. The clinical presentation in humans follows the Fortaleza Classification as the stage of infection will determine the symptoms present. The following discussion will give an overview of the symptoms beginning in stage 2 because patients are not likely to present themselves at the early stages of infection, mostly because the flea’s burrowing is usually not felt. This may be due to a keratolytic enzyme secreted during stage 1.

The patient with a single flea may present as early as stage 2 when, though the erythema is barely perceptible, a boring pain and the curious sensation of pleasant itching occur. This inflammatory reaction is the initial immunological response to the infestation. Heavily infested patients may not notice a stage 2 infection due to the other fleas’ causing irritation as well. Feces may be seen, but this is more common in the 3rd stage.

Around the third day after penetration, erythema and skin tenderness are felt, accompanied by pruritus (severe itching) and a black furuncular nodule surrounded by a white halo of stretched skin caused by the expansion of the flea. Fecal coils may protrude from the center of the nodule where the flea’s anus is facing upward. They should be washed off quickly as the feces may remain in the skin unless removed. During this 3a substage, pain can be severe, especially at night or, if the nodule is on the foot, while walking. Eggs will also begin to be released and a watery secretion can be observed. The radical metamorphosis during the 3rd to 6th day after penetration, or neosomy, precedes the formation of a small caldera-like rim rampart as a result of the increased thickness of the flea’s chitin exoskeleton. During the caldera formation, the nodule shrinks a bit and it looks as if it is beginning to dry out; this takes 2 weeks and comprises substage 3b.

At the third week after penetration and substage 4a, the eggs’ release will have stopped and the lesion will become smaller and more wrinkled. As the flea is near death, fecal and water secretion will stop altogether. Pain, tenderness, and skin inflammation will still be present. Around the 25th day after penetration, the lesion looks like a black crust and the flea’s carcass is removed by host repair mechanisms and the skin begins to heal. With the flea gone, inflammation may still persist for a while.

Although patients would not present within the 5th stage of tungiasis as the flea would be dead and no longer in the body, this stage is characterized by the reorganization of the skin (1–4 weeks) and a circular residue of 5–10 mm in diameter around the site in penetration. An intraepithelial abscess, which developed due to the presence of the flea, will drain and later heal. Although these disease residues would persist for a few months, tungiasis is no longer present.

In severe cases, ulcers are common, as well as complete tissue and nail deformation. A patient may be unable to walk due to severe pain if too many of the lesions are present in the feet. Suppuration (pus formation), auto-amputation of digits (via ainhum), and chronic lymphedema may also be seen.

If the patient is not vaccinated, tetanus is often a complication due to secondary infection. Gangrene is another common complication of severe infestation and superinfection. Staphylococcus aureus and Wolbachia endobacteria can be transmitted by the chigoe flea, as well as nearly 150 other different pathogens. For these reasons, the chigoe flea should be removed as soon as possible.

Skeletal eroding band (SEB) is a disease of corals that appears as a black or dark gray band that slowly advances over corals, leaving a spotted region of dead coral in its wake. It is the most common disease of corals in the Indian and Pacific Oceans, and is also found in the Red Sea.

So far one agent has been clearly identified, the ciliate "Halofolliculina corallasia". This makes SEB the first coral disease known to be caused by a protozoan. When "H. corallasia" divides, the daughter cells move to the leading edge of the dark band and produce a protective shell called a lorica. To do this, they drill into the coral's limestone skeleton, killing coral polyps in the process.

A disease with very similar symptoms has been found in the Caribbean Sea, but has been given a different name as it is caused by a different species in the genus "Halofolliculina" and occurs in a different type of environment.

MRLS was initially characterized by four syndromes: (1) EFLs, (2) LFLs and the nonreproductive syndromes, (3) unilateral uveitis, (4) pericarditis, and later (5) "Actinobacillus" encephalitis. MRLS was observed in mares of all breeds and ages. Early and late fetal losses were observed within the first and last trimesters of pregnancy, respectively. For EFLs, clinical signs from the sick mares included pus-like discharge from the vulva and fetal membranes protruding from the vulva, as the fetuses were located in either the vagina or vulva. One to three days prior to the EFLs, several mares showed mild colic symptoms, abdominal straining, or low-grade fever. Within a week of abortion, inspections indicated moderate to severe inflammation within the uterine region. Performing ultrasounds revealed either dead fetuses or live fetuses with slow heart rates and lethargic movements. All the fetuses, both alive and dead, were surrounded by cloudy amniotic fluid.

Clinical signs for LFLs included explosive parturition, dystocia, foaling while standing, premature placenta separation, and foals either stillborn or born weak. Placentas were observed to carry a pale brown hue as opposed to their usual dark reddish-brown color. The umbilical cords were thick, dull, yellowish, and inflamed. The weak foals were often incapable of breathing on their own and required resuscitation. These foals were also observed to be dehydrated and hypothermic, with irregular heartbeat and respiration. The majority of these foals did not survive past four days.

One finding observed exclusively in MRLS was unilateral uveitis; initially, affected horses expressed inflammation around a single eye, along with fluid accumulation on the corneas, anterior and posterior chambers due to the inflammation. The fluid in the anterior chambers exhibited tan to yellow hues, and often was accompanied by hemorrhaging around the surface of the iris.

"Rabbits, Hares & Lagomorphs"

Usually there do not appear to be any clinical signs. Subcutaneous cysts, warbles, may present upon larval deposition out of the body at maturation.

"Felines & Canines"

There are three forms in which Cuterebriasis may present:

- Myasis

- Cerebrospinal

- Respiratory

Myasis involves subcutaneous cyst formation due to 3rd larval instar maturation, occurring ~30 days post-entry into the body. Cysts are often found on the face, neck and trunk, but location varies with larval migration within the host. Serous discharge may be observed from these cysts, which are typically 3-5mm in diameter and include a central pore through which the larvae respire. This pore also serves as a means of exit for the larvae, which occurs anywhere between 3 and 8 weeks post-entry.

Cerebrospinal cuterebriasis results from larval migration to the brain. This is seen in cats, and is the proposed cause for feline ischemic encephalopathy and a suggestive causative agent of feline idiopathic vestibular disease. Symptoms of this type of presentation include lethargy, seizures, blindness, abnormal vocalization or gait, circling, and abnormal or no reflex responses. When affecting the central nervous system, cats are known to exhibit violent sneezing attacks that can onset weeks prior to manifestation of other clinical signs.

Respiratory disease results when larval migration occurs through the trachea, pharynx, diaphragm, or lungs. Cuterebriasis has been increasingly noted as a cause for dyspnea in felines.

Symptoms of an ingrown nail include pain along the margins of the nail (caused by hypergranulation that occurs around the aforementioned margins), worsening of pain when wearing tight footwear, and sensitivity to pressure of any kind, even the weight of bedsheets. Bumping of an affected toe can produce sharp and even excruciating pain as the tissue is punctured further by the nail. By the very nature of the condition, ingrown nails become easily infected unless special care is taken early to treat the condition by keeping the area clean. Signs of infection include redness and swelling of the area around the nail, drainage of pus and watery discharge tinged with blood. The main symptom is swelling at the base of the nail on the ingrowing side (though it may be both sides).

Onychocryptosis should not be confused with a similar nail disorder, convex nail, nor with other painful conditions such as involuted nails, nor with the presence of small corns, callus or debris down the nail sulci (grooves on either side).

There are two different manifestations of rainscald: the winter form, which is more severe due to the longer coat of the horse, and the summer form, which is less severe. Horses are usually affected on the back, head, and neck, where insects commonly bite, and the legs, which are commonly infected if the horse is kept in moist footing. Initially, the horse will display a matted coat and bumps which will then progress to crusty scabs and lesions. The animal may also be itchy and display signs of discomfort.

A needlestick injury, percutaneous injury, or percutaneous exposure incident is the penetration of skin by a needle or other sharp object, which was in contact with blood, tissue, or other body fluid before the exposure. Occupational needlestick injuries primarily affect healthcare workers, who make up 80% of needlestick injuries in the United States. Various other occupations are also at increased risk of needlestick injury, including law enforcement, laborers, tattoo artists, food preparers, and agricultural workers. Though the acute physiological effects of a needlestick injury are generally negligible, these devices can transmit blood-borne diseases, placing those exposed at increased risk of contracting infectious diseases, such as hepatitis B (HBV), hepatitis C (HCV), and the human immunodeficiency virus (HIV). Among healthcare workers and laboratory personnel worldwide, more than 25 blood-borne viruses have been reported to be caused by needlestick injuries.

It is estimated that half of all occupational needlestick injuries are not reported. Additionally, an unknown number of occupational needlestick injuries are reported by the affected employee, yet due to organizational failure, institutional record of the injury does not exist. Increasing recognition of the unique occupational hazard posed by needlestick injuries, as well as the development of efficacious interventions to minimize the largely preventable occupational risk, encouraged legislative regulation in the US, causing a decline in needlestick injuries among healthcare workers.

Because of the relatively rapid onset of tungiasis, the incubation period tends to be short. Although some reddening around the site of penetration occurs, the first symptoms are perceived in stage 2 as itching and severe pain, usually a day after penetration.

Eyespot is an important fungal disease of wheat caused by the necrotrophic fungus Tapesia yallundae (syn: "Pseudocercosporella herpotrichoides"; W-type [anamorph]; "Oculimacula yallundae") and Tapesia acuformis (syn: "Pseudocercosporella herpotrichoides"; R-type [anamorph]; "Oculimacula acuformis"). It is also called Strawbreaker. Eyespot is more severe where wheat is grown continuously and when the weather is cool and moist. Treating crops against eyespot with fungicide costs millions to farmers and is complicated by the pathogen becoming resistant to the more commonly used fungicides. Severe cases of the disease can reduce yield by up to 40%. It is most common in temperate regions such as North and South America, Europe, Australia, New Zealand and Africa.

An ingrown nail (also known as onychocryptosis from ὄνυξ ("onyx", "nail") + κρυπτός ("kryptos", "hidden") or unguis incarnates) is a common form of nail disease. It is an often painful condition in which the nail grows so that it cuts into one or both sides of the paronychium or nail bed.

The common opinion is that the nail enters inside the paronychium, but an ingrown toenail can simply be overgrown toe skin. The condition starts from a microbial inflammation of the paronychium, then a granuloma, which results in a nail buried inside of the granuloma. While ingrown nails can occur in the nails of both the hands and the feet , they occur most commonly with the toenails.

A true ingrown toenail is caused by the actual penetration of flesh by a sliver of nail.

Diagnosis is most commonly done with the identification of bacteria in the lesions by a microscope observation. Ticks, biting flies, and contact with other infected animals also causes the spread of rainscald. A scab will be taken from the affected animal and stained so that the bacteria are visible under a microscope inspection. A positive diagnosis of rainscald can be confirmed if filamentous bacteria are observed with as well as chains of small, spherical bacteria. If a diagnosis cannot be confirmed with a microscope, blood agar cultures can be grown to confirm the presence of "D. congolensis". The resulting colonies have filaments and are yellow in colour.

A popping or cracking sound, significant pain, swelling, immediate loss of erection leading to flaccidity, and skin hematoma of various sizes are commonly associated with the sexual event.

Mare reproductive loss syndrome (MRLS) is a syndrome consisting of equine abortions and three related nonreproductive syndromes which occur in horses of all breeds, sexes, and ages. MRLS was first observed in the U.S. state of Kentucky in a three-week period around May 5, 2001, when about 20% to 30% of Kentucky's pregnant mares suffered abortions. A primary infectious cause was rapidly ruled out, and the search began for a candidate toxin. No abortifacient toxins were identified.

In the spring of 2001, Kentucky had experienced an extraordinarily heavy infestation of eastern tent caterpillars (ETCs). An epidemiological study showed ETCs to be associated with MRLS. When ETCs returned to Kentucky in the spring of 2002, equine exposure to caterpillars was immediately shown to produce abortions. Research then focused on how the ETCs produced the abortions. Reviewing the speed with which ETCs produced late-term abortions in 2002 experiments, the nonspecific bacterial infections in the placenta/fetus were assigned a primary driving role. The question then became how exposure to the caterpillars produced these non-specific bacterial infections of the affected placenta/fetus and also the uveitis and pericarditis cases.

Reviewing the barbed nature of ETC hairs (setae), intestinal blood vessel penetration by barbed setal fragments was shown to introduce barbed setal fragments and associated bacterial contaminants into intestinal collecting blood vessels (septic penetrating setae). Distribution of these materials following cardiac output would deliver these materials to all tissues in the body (septic penetrating setal emboli). About 15% of cardiac output goes to the late-term fetus, at which point the septic barbed setal fragments are positioned to penetrate placental tissues which lack an immune response. Bacterial proliferation, therefore, proceeds unchecked and the late-term fetus is rapidly aborted.

Similar events occur with the early-term fetus, but as a much smaller target receiving an equivalently smaller fraction of cardiac output, the early-term fetus is less likely to be "hit" by a randomly distributing setal fragment. Since this MRLS pathogenesis model was first proposed in 2002, other caterpillar-related abortion syndromes have been recognized, most notably equine amnionitis and fetal loss in Australia, and more recently, a long-recognized relationship between pregnant camels eating caterpillars and abortions among the camel pastoralists in the western Sahara.

An early stage of hyperthermia can be "heat exhaustion" (or "heat prostration" or "heat stress"), whose symptoms include heavy sweating, rapid breathing and a fast, weak pulse. If the condition progresses to heat stroke, then hot, dry skin is typical as blood vessels dilate in an attempt to increase heat loss. An inability to cool the body through perspiration may cause the skin to feel dry.

Other signs and symptoms vary. Accompanying dehydration can produce nausea, vomiting, headaches, and low blood pressure and the latter can lead to fainting or dizziness, especially if the standing position is assumed quickly.

In severe heat stroke, there may be confused, hostile, or seemingly intoxicated behavior. Heart rate and respiration rate will increase (tachycardia and tachypnea) as blood pressure drops and the heart attempts to maintain adequate circulation. The decrease in blood pressure can then cause blood vessels to contract reflexively, resulting in a pale or bluish skin color in advanced cases. Young children, in particular, may have seizures. Eventually, organ failure, unconsciousness and death will result.

In humans, hyperthermia is defined as a temperature greater than , depending on the reference used, that occurs without a change in the body's temperature set point.

The normal human body temperature can be as high as in the late afternoon. Hyperthermia requires an elevation from the temperature that would otherwise be expected. Such elevations range from mild to extreme; body temperatures above can be life-threatening.

Bumblefoot (ulcerative pododermatitis) is a bacterial infection and inflammatory reaction on the feet of birds, rodents, and rabbits. Ulcerative pododermatitis is referred to as "sore hocks" when it affects a rabbit and "bumblefoot" when it affects a bird. The terms "sore hocks" and "bumblefoot" are used interchangeably when describing ulcerative pododermatitis in rodents. The infection can usually be attributed to poor husbandry practices, therefore is much more likely to occur in captive animals than in those in the wild. It is caused by bacteria, namely strains of "Staphylococcus", "Pseudomonas" and "Escherichia coli" "(E. coli)", with "S. aureus" being the most common cause of the infection.

The eye-shaped elliptical lesions which give eyespot its name form on lower stem bases near to the soil surface. The lesions are straw yellow, often with black pupil-like dots in the centre, and are bordered by greenish-brown to dark-brown rings. In cases of severe infection stems are weakened at the point of infection which makes the host susceptible to lodging. This symptom is associated with the W-type (Oculimacula yallundae). Alternatively in other cases of severe infection the nutrient and water supply to the plant is disturbed, resulting in low quality grain and whitehead production due to early maturation. This is associated with the R-type (Oculimacula acuformis)

Penile fracture is rupture of one or both of the "tunica albuginea", the fibrous coverings that envelop the penis's "corpora cavernosa". It is caused by rapid blunt force to an erect penis, usually during vaginal intercourse, or aggressive masturbation. It sometimes also involves partial or complete rupture of the urethra or injury to the dorsal nerves, veins and arteries.

The use of podiatry drills, in the absence of engineering controls and personal protective equipment, is an occupational hazard to the healthcare provider. Nail dust collected during foot care procedures performed in office settings has been found to contain keratin, keratin hydrolysates, microbial debris, and viable fungal elements, including dermatophytes (most commonly "Trichophyton rubrum") and saprotrophs. Exposure to nail dust and the associated risk will vary with the policies and practices in place, the type of podiatry drill used, therapy technique, frequency of procedures, personal protective equipment utilized and the use of ventilation systems.

Bumblefoot is, perhaps, the largest cause of referral of birds of prey to a veterinary surgeon. Bumblefoot on birds of prey can be put into three broad types of the infection.

In the first type, a small reddened area, or sometimes a small shiny patch, can be seen on the foot. This is mostly caused by inappropriate perching (or perching for too long), or less likely, by badly fitted furniture, such as jesses that are too small. To treat this type, one must change the fault in the husbandry, fly the bird regularly, and apply hemorrhoid cream to the affected area.

The second type is more serious, where some penetration has occurred. While treatment for the first type will help, the bird likely will require antibiotics, as well.

The third type involves the bird having severe distortion of the contours of the foot and/or the toes, resulting from the bumblefoot causing considerable damage in the foot.

Blast-related ocular trauma comprises a specialized group of penetrating and blunt force injuries to the eye and its structure caused by the detonation of explosive materials. The incidence of ocular trauma due to blast forces has increased dramatically with the introduction of new explosives technology into modern warfare. The availability of these volatile materials, coupled with the tactics of contemporary terrorism, has caused a rise in the number of homemade bombs capable of extreme physical harm.

Strawberry foliar nematode is a disease common in strawberries and ornamental plants that can greatly affect plant yield and appearance, resulting in a loss of millions of dollars of revenue. Symptoms used to diagnose the disease are angular, water soaked lesions and necrotic blotches. "Aphelenchoides fragariae" is the nematode pathogen that causes the disease. Its biological cycle includes four life stages, three of which are juvenile. The nematode can undergo multiple life cycles in one growing season when favorable conditions are present. They can infect the crowns, runners, foliage, and new buds of the plant via stylet penetration or through the stomata. The best management practices for this disease are sanitation, prevention of induction of the pathogen to the environment, and planting clean seed or starter plants.