Trichophyton rubrum fungus. Computer illustration showing macroconidia, microconidia and septate hyphae. T. rubrum is an anthropophilic dermatophyte. Dermatophyte fungi infect keratinized tissues (skin, nails, hair) in humans and other animals. The downy strain of T. rubrum has become the most widely distributed dermatophyte of humans. It frequently causes chronic infections of the skin and nails and rarely of the scalp. The thread-like structures are hyphae, the vegetative filamentous cells of this fungus. This hyphal stage is the saprophytic stage of the fungus. Fungal growth is usually confined to tips of hyphae, where asexual reproduction occurs by spore formation. Dermatophytes preferentially inhabit the nonliving, cornified layers of the skin, hair, and nail, which is attractive for its warm, moist environment conducive to fungal growth. Other species of Trichophyton can cause disease in humans, inhabiting the skin, nails and scalp.