Influenza pandemics

The swine flu outbreaks cropping up in several countries now are nothing — so far — compared to historical flu outbreaks. But with more people on the planet, and more of them huddled in cities and more of them traveling so easily, the potential for pandemic is not lost on health officials.

Flu history is frightening: The 1918 influenza pandemic swept the world within months, killing an estimated 50 million people — more than any other illness in recorded history for the short time frame involved.


One-fifth of the world's population was infected, and it struck more than 25 percent of U.S. residents. Unlike some flu strains that mainly kill the elderly, children, and those with compromised immune systems, the 1918 strain hit young adults hard. In one year, the average life expectancy in the United States dropped by 12 years.


Today, governments are more prepared, scientifically and logistically, to handle flu outbreaks. Still, there is no vaccine for swine flu, and it could take months, or more, to develop one.

Diseases that bite

A range of zoonotic diseases — thought to be on the rise — are caused by animal bites that kill hundreds of thousands of people every year. Mosquitoes lead the way: Malaria infects 350 million or more people every year, and more than 1 million die, most of them young children in Africa south of the Sahara. Mosquito-borne dengue fever infects some 50 million people annually; about 500,000 are hospitalized and about 2.5 percent of those die.

The problem is growing, with insect-borne disease outbreaks becoming more common and more virulent, and epidemics having spread to the Americas. Scientists say the warming climate will only make matters worse.


Illustrating our illness connection to animals and especially pets, rabies kills about 55,000 people globally each year, mostly in Asia and Africa. Most deaths follow a bite from an infected pet dog, though wild animals can carry rabies too. And an estimated 16 million or more people from Mexico to Argentina are affected by Chagas disease, a chronic, frequently fatal infection transmitted by the feces of blood-feeding bugs called triatomines (commonly called "kissing bugs"). Chagas is often spread by dogs or even chickens that are kept indoors at night, giving the bugs access to the people.


You don't even have to be bitten by bugs or animals to get some deadly diseases from them. Hantavirus is carried mostly by deer mice. You can catch it by breathing dust contaminated with mouse droppings. Lack of appetite, fever, vomiting and muscle aches characterize Phase 1. Phase 1? Yep. Just when you start to feel better, you may get a stiff neck, another fever, confusion and have trouble moving. It is incurable, but most people recover. About 1 percent die.

Insane mind parasite

The bizarre parasite Toxoplasma gondii infects the brains of more than half the human population, including about 50 million Americans. It is thought increase the risk of neuroticism and may contribute to schizophrenia.

However, its primary host is house cats, in which the microbe reproduces sexually. Cats left to roam are more prone to picking it up. You can get it from cat feces. The bug is also found in many other mammals, too (where it reproduces asexually).


Initially, symptoms in humans are typically flu-like. But this bug never goes away. Some scientists think it has altered human behavior enough to shape entire cultures. Countries with high prevalence of T. gondii infection also have higher average neuroticism scores, one study found.

Ebola

Ebola is a widespread threat to gorillas and chimps in Central Africa, and may have spread to humans from people who ate infected animals. It is now transmittable human-to-human, by contact with the blood or body fluids of an infected person, and it has killed a few hundred people in each of several outbreaks going back to the mid-1970s.

The awful symptoms: sudden onset of fever, intense weakness, muscle pain, headache and sore throat, often followed by vomiting, diarrhea, rash, impaired kidney and liver function, and in some cases, both internal and external bleeding. It's deadly somewhere between 25 and 90 percent of the time depending on the strain.


Ebola may be carried by bats, researchers think, because when bats are infected with it, they don't die.

Human virus that kills chimps

Ecotourism fuels outbreaks of respiratory diseases among African chimps. The human respiratory syncytial virus (HRSV) and human metapneumovirus (HMPV) kill infant humans in developing countries. Nearly all humans have had contact with these germs, though, and so have developed antibodies naturally designed to fight them. But in the first confirmed evidence of viruses transmitted directly from humans to wild great apes, the virus killed entire populations of chimps in part of West Africa between 1999 and 2006.