Particles must be small to penetrate deep into human lungs, where they can start a lethal infection.

Aspergillus, further, was known to cause lethal infections.

These observations raise questions about mechanisms that favor the survival of most children in Africa while allowing some to develop potentially lethal infections.

The microsporidia often cause chronic, debilitating diseases rather than lethal infections.

This situation worries a growing number of experts, who fear that improperly handled tissue can transmit dangerous or even lethal infections.

The drugs weakened the patients' immune systems, often leading to lethal infections.

The disease cripples the immune system, leaving the victim susceptible to lethal infections and cancers.

A lethal infection is reported to result from inhalation of about 10,000-20,000 spores, though this dose varies among host species.

It takes an estimated 8,000 to 10,000 anthrax germs lodged in the human lung to start a lethal infection.

Why would anyone knowingly expose himself to a potentially lethal infection?