This article is intended principally with the traveller in jungle in mind, although it applIes to any subtropical non-urban climate you might have the pleasure of travelling through.
We start with a list of potential attackers, and then we will list some generic avoidance measures. This author has been attacked by all the creatures we include in our list, so this article is based principally on first hand experience.
Almost all the attacks were in Central or South America; some were inflicted in central Africa. These are without doubt the two most dangerous parts of the world when it comes to small toxic animals.
Very few of these creatures are fatal if treated properly. But they are all extremely unpleasant.
If you are affected, seek local treatment. You are very unlikely to find a doctor back home that has the slightest idea what the ailment is likely to be, let alone who knows how to treat it. Tropical medicine is a very rare speciality in most non-tropical countries. You could spend months trying to find the right person.
Botflies
Botflies themselves are large flying insects that live as cattle parasites. They will not harm you.
However the problem comes when you are bitten by a mosquito that carries botfly larvae. Mosquitos are common carriers.
Then the larva grows in your body, at the place where you were stung but just under the skin; and it keeps a hole open in your skin out of which to defecate while it is growing into a full botfly.
The result is quite disgusting: streams of excrement running out of little holes in your body amidst great stinging pain.
Ultimately humans are bad hosts, so if the larva remains unremoved then eventually it explodes under your skin, creating a large infected crater that takes weeks or months to heal.
The treatment, when you have what look like red sores on your body that periodically leak excrement - is to find a specialist doctor who will remove the botflies using a tub of vaseline. The doctor covers the breathing / excreting holes so the larva must stick its proboscis out of the hole to move away the vaseline so it can breathe. If you keep putting more and more vaseline on the hole, eventually the larva's head pops out. Then with a combination of tweezers and lots of squeezing of the surrounding skin, the botfly pops out of the body.
If you just pull on the head, the larva breaks inside you, creating a deep infection.
It is common to have multiple larvae, often in one's back. This author had four. It took about six hours for a Mexican doctor to remove them.
Once they are removed, the holes close up promptly and if the removal is undertaken effectively, there is no residual infection. You just walk away.
Tsetse flies
These alarmingly large, slow-moving flies are a sight to behold. They can happily chase a person round the bedroom. A fly swat will not defeat them; they are too armoured and tough. To kill them, resort to a cigarette lighter and an aerosol can to create an impromptu flame thrower. Their wings will burn and they will fall to the ground. Then stamp on them repeatedly with a boot.
They are mostly found only in West Africa.
If you are bitten, they carry the disease trypanosomiasis (sleeping sickness), which is fatal without treatment. Treatment involves a complex drug cocktail that should only be undertaken under the supervision of an experienced doctor in tropical diseases. Most doctors in West Africa know how to treat it; the reason people die is usually because they cannot afford the medications.
Mosquitos and malaria
There is a band of equatorial countries where malaria is a risk. Within that band, malaria is invariably passed by mosquito bites.
Malaria involves a long, tedious course of treatment if you contract it. The proper course is prophylaxis if you are entering a malarial area. The prophylactic is doxycycline 100mg daily, starting two days before entering the affected area; throughout one's period in the affected area; and for 28 consecutive days after leaving it. Under such a regimen, you will not contract malaria. But be accurate and precise; do not miss days out or take the pills at different times of the day.
Poisonous snakes
Death by snake is exceptionally rare. Few snakes attack unprovoked. You might stand on one accidentally; that is why you should wear solid boots.
Here is a list of some of the most deadly snakes. They are all extremely rare.
If you are bitten by a snake, cut off its head with a knife or pair of scissors and then take both parts of the carcass to a specialist in tropical illnesses who will prescribe an appropriate anti-venom if necessary. It is very hard for them to do that if they do not have the snake and they are having to guess.
To repeat: the greater majority of snakes are not aggressive nor are their bites toxic. If you see one, just stay away from it.
UPDATE 23 May 2022
We are informed that Australian medical authorities have created a generalised anti-venom that works against all snake bites. Therefore it is not as important, if bitten in Australia, to take the snake with you when seeking medical attention.
Parasitic worms
There are three principal categories of parasitic worms to be aware of: ring worms, hook worms and tape worms. All are extremely unpleasant.
Ring worms are not really worms at all but fungal infections. The infection can spread and mutate across the skin. When you see fungal rings on your body, seek anti-fungal treatment immediately. You may need an extended course of treatment to rid yourself of quite an unpleasant phenomenon.
Hook worms are minute creatures that infect your digestive tract and your intestines in particular. You get them by eating contaminated meat, for example. They can cause widespread infection and disease in the organs. They are treated with a short course of specialist medications; with the right diagnosis, you will be free of them within a few days.
Tape worms also start life as microscopic parasites present in bad food. They also attach themselves to the intestines; and then they grow with gradual but increasingly unpleasant symptoms.
The traditional treatment for tape worms in some areas has been to wait until the worm grows so long that it reaches the skin: then to cut open the skin and slowly pull the tape worm out by wrapping it round a pencil or similar object and then twisting the pencil so that the tape worm is gradually extracted from the body.
Do not suffer anyone who suggests this course of treatment. As well as being disgusting, it is bad medicine. The worm might well break during the extraction process, leaving a dead worm in your digestive tract that is bound to lead to infection requiring extended treatment with complex and strong antibiotics.
Rather, the same categories of medicine that kill hook worms also kill tape worms. Nevertheless diagnosis is difficult and requires a tropical medicine specialist. You are much more likely to find one of these in the tropics.
Bedbugs
Bed bugs are invisible to the naked eye. They live in unsanitary conditions, looking for targets to bite and suck their blood. Hence the name bedbug: they are often present in unclean bedsheets or other bed linen.
The way you know you are the victim of bedbugs is that you wake up in the morning and there is a neat row of small bloodstains down the sheets. The bedbugs have spent the night dining, and the meal was you.
Obviously the first thing to do is to change hotels. This author contracted bedbugs in what was at the time the most expensive hotel in Dar Es-Salaam. Things may only be superficially clean. You should also take a short course of broad spectrum antibiotics (e.g. ciprofloxacin 500mg 1 x day for three days) to foreclose any infections.
Scorpions
This author once woke up, sleeping in a camp in lowland Bolivian jungle, to find a large scorpion in his underwear. Nevertheless he had little to fear. The venom of scorpions lies in inverse proportion to the scorpion's size. Big ones are mostly harmless. Small ones can be highly toxic.
If you are stung, capture the scorpion to show to a tropical diseases specialist. Scorpions are very rarely fatal.
Triatominae (vampire bugs)
These are insects that climb onto your face in the middle of the night and suck its blood, leaving bloody spots on the face and likely infections, including Chagas Disease, an unpleasant parasitic disease that requires a special combination of anti-parasitic drugs and antibiotics to cure.
Triatominae are almost always the consequence of dirty hotels and dirty bed linen in tropical countries.
Poisonous caterpillars
As with many creatures in the categories identified in this article, most are not poisonous and the ones that are tend to be very small.
Poisonous caterpillars are a particular phenomenon of the Eastern United States, where doctors will know how to treat toxic caterpillar stings. They are very rarely deadly. Just take the caterpillar to the medical specialist.
A full list of them is here, but the general rule is not to allow them to crawl into your shoes or socks at night.
https://www.bioexplorer.net/poisonous-caterpillars.html/
Spiders
This author once woke up in his bed in a hotel in central Ethiopia to find a tarantula crawling past his head. It was quite a shock. It probably did not help that the ensuite facilities were a pile of rubble and faeces. Nevertheless tarantulas are mostly harmless. They are very large, which makes one shocked or horrified. But they rarely bite. And if they do, their bite is no worse than a bee's sting.
There are toxic spiders; they are all extremely small. Here is a list:
It is exceedingly rare to be bitten by a toxic spider. If you are, preserve the spider (after you have killed it) and take yourself and the carcass to a tropical medicine specialist.
Leeches
Leeches are parasites that sink their jaws or proboscises into your flesh and suck your blood, injecting a peptide to prevent clotting.
They are found for the most part in dirty water. They have no medicinal value but they do cause unpleasant infections.
If you have a leech, burn it off with a cigarette lighter (do not pull it off or its teeth will remain in your skin, causing infection) and then seek medical attention (taking the leech with you), most likely a course of antibiotics to suppress infection.
Phlebotominae (sandflies)
These are blood sucking flies, found in the Americas and the Caribbean, parts of west Africa and in Australasia. The swelling they cause usually subsides within a few days. However they can carry all sorts of tropical diseases, including a version of rabies. They can also carry visceral leishmaniasis (also known as Kala Azar), a deadly disease that is very hard to treat.
Sandflies are generally very bad news, and you should do everything possible to prevent being bitten by them.
Miscellaneous diseases
Although we cannot provide comprehensive advice on all tropical diseases, not passed by small animals and insects, here is a list of other tropical diseases you may contract by means of minute organisms not visible to the naked eye. You should familiarise yourself with all of them.
Plague (bacterium)
Yellow fever (virus)
Scarlet fever (bacterium)
Meningo-encephalitis (fungus)
Bacillic and amoebic dysentry (bacterium)
Each of these illnesses is treated in different ways (bacillic dysentry with ciprofloxacin; amoebic dysentry with metronidazole, for example). You should be aware of the symptoms of each (they are described in detail on Wikipedia entries) and get yourself to a tropical diseases specialist if you find yourself with any of the symptoms of them.
Generic avoidance mechanisms
All the above creatures are difficult to avoid entirely - they are just out there, and there is no way of getting rid of them. Nevertheless there are things you can do to minimise the possibility of attack.
Ensure adequate sleeping facilities. Sleep in clean hotels. If you are sleeping outdoors, sleep in a hammock with a mosquito net. Do not sleep on the jungle floor in any circumstances!
Wear proper shoes (boots). It does not matter how hot it is. You want tough boots that reach up your ankles.
Carry a large can of aerosol-based tropical insecticide, either with DEET or a general insecticide. Spray everything with it: your clothes, your bed linen, your mosquito net, your hammock, your boots, your face and hair.
Check your socks and shoes before putting them on each time. In the jungle, sleep with your boots on.
Cover your skin. Again it does not matter how hot it is. Covered skin, with clothes regularly rinsed in insecticide, is the best defence to virtually all these things.
Stay clean. Try to wash as often as you can, and then spray yourself with insecticide after you are dry.
Be extremely careful what you eat. Insist that meat is thoroughly cooked. If it is not 'well done', send it back to the kitchen. Do not eat soups or drink things that may have come from a water tap or open water source without being certain they have been thoroughly boiled. Do not eat salads or other uncooked vegetables. Soft drinks, beer and wine are fine but ensure you see the cap lid or cork being removed in front of you. Under no circumstances eat anything 'raw'. Avoid things like Ethiopian raw liver - a delicacy perhaps, but not a wise one to try. (This author contracted hook worms from eating that dish.) Generally, Africa is the safest place in the tropics for properly cooked food; rural parts of the Indian subcontinent are the worst.
Travelling in basic conditions in tropical environments, it is almost inevitable that one of these things gets you. Be prepared, and receive treatment locally. It will be a fraction of the cost and far more likely to involve an accurate diagnosis and treatment.